diff --git a/change/@itwin-imodel-transformer-29a95bab-526a-4bfb-805c-b895d6666c8a.json b/change/@itwin-imodel-transformer-29a95bab-526a-4bfb-805c-b895d6666c8a.json
new file mode 100644
index 00000000..ccdb3907
--- /dev/null
+++ b/change/@itwin-imodel-transformer-29a95bab-526a-4bfb-805c-b895d6666c8a.json
@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "Fixed BigMap class to implement Map (instead of extending it). Added missing method implementations.",
+  "packageName": "@itwin/imodel-transformer",
+  "email": "Julija.Ramoskiene@bentley.com",
+  "dependentChangeType": "patch"
+}
diff --git a/packages/transformer/src/BigMap.ts b/packages/transformer/src/BigMap.ts
index 6554ed0a..d311f779 100644
--- a/packages/transformer/src/BigMap.ts
+++ b/packages/transformer/src/BigMap.ts
@@ -14,16 +14,15 @@ import { Id64String } from "@itwin/core-bentley";
  * replace this stopgap with a more robust solution, utilizing a temporary SQLite database (https://github.com/iTwin/imodel-transformer/issues/83).
  * @internal
  */
-export class BigMap<V> extends Map<Id64String, V> {
+export class BigMap<V> implements Map<Id64String, V> {
     private _maps: Record<string, Map<string, V>>;
     private _size: number;
 
-    public override get size(): number {
+    public get size(): number {
       return this._size;
     }
 
     public constructor() {
-      super();
       this._maps = {
         0: new Map(),
         1: new Map(),
@@ -45,12 +44,12 @@ export class BigMap<V> extends Map<Id64String, V> {
       this._size = 0;
     }
 
-    public override clear(): void {
+    public clear(): void {
       Object.values(this._maps).forEach((m) => m.clear());
       this._size = 0;
     }
 
-    public override delete(key: Id64String): boolean {
+    public delete(key: Id64String): boolean {
       const wasDeleted = this._maps[key[key.length - 1]].delete(key);
       if (wasDeleted) {
         this._size--;
@@ -59,21 +58,21 @@ export class BigMap<V> extends Map<Id64String, V> {
       return wasDeleted;
     }
 
-    public override forEach(callbackfn: (value: V, key: Id64String, map: Map<Id64String, V>) => void, thisArg?: any): void {
+    public forEach(callbackfn: (value: V, key: Id64String, map: Map<Id64String, V>) => void, thisArg?: any): void {
       Object.values(this._maps).forEach((m) => {
         m.forEach(callbackfn, thisArg);
       });
     }
 
-    public override get(key: Id64String): V | undefined {
+    public get(key: Id64String): V | undefined {
       return this._maps[key[key.length - 1]].get(key);
     }
 
-    public override has(key: Id64String): boolean {
+    public has(key: Id64String): boolean {
       return this._maps[key[key.length - 1]].has(key);
     }
 
-    public override set(key: Id64String, value: V): this {
+    public set(key: Id64String, value: V): this {
       const mapForKey = this._maps[key[key.length - 1]];
       if (mapForKey === undefined)
         throw Error(`Tried to set ${key}, but that key has no submap`);
@@ -83,4 +82,36 @@ export class BigMap<V> extends Map<Id64String, V> {
       this._size += (afterSize - beforeSize);
       return this;
     }
+
+    public [Symbol.iterator](): IterableIterator<[Id64String, V]>{
+      return this.entries();
+    }
+
+    public get [Symbol.toStringTag]() {
+      return "BigMap";
+    }
+
+    public *entries(): IterableIterator<[Id64String, V]> {
+      const maps = Object.values(this._maps);
+      for (const map of maps) {
+        for (const [key, value] of map.entries())
+          yield [key, value];
+      }
+    }
+
+    public *keys(): IterableIterator<Id64String> {
+      const maps = Object.values(this._maps);
+      for (const map of maps) {
+        for (const key of map.keys())
+          yield key;
+      }
+    }
+
+    public *values(): IterableIterator<V> {
+      const maps = Object.values(this._maps);
+      for (const map of maps) {
+        for (const value of map.values())
+          yield value;
+      }
+    }
 }
diff --git a/packages/transformer/src/test/standalone/BigMap.test.ts b/packages/transformer/src/test/standalone/BigMap.test.ts
new file mode 100644
index 00000000..b46f2ca5
--- /dev/null
+++ b/packages/transformer/src/test/standalone/BigMap.test.ts
@@ -0,0 +1,84 @@
+/*---------------------------------------------------------------------------------------------
+* Copyright (c) Bentley Systems, Incorporated. All rights reserved.
+* See LICENSE.md in the project root for license terms and full copyright notice.
+*--------------------------------------------------------------------------------------------*/
+
+import { BigMap } from "../../BigMap";
+import { assert } from "chai";
+
+describe("BigMap", function () {
+
+  // Test map keys will be assigned into 2 different submaps when BigMap is created
+  const testMap = new Map ([["0x123f", "testVal1"], ["0x1231", "testVal2"]]);
+
+  const createBigMap = (map: Map<string, string>): BigMap<string> => {
+    const bigMap = new BigMap <string> ();
+    for (const entry of map.entries ()) {
+      bigMap.set(entry[0], entry[1]);
+    }
+    return bigMap;
+  };
+
+  describe("keys", function () {
+    it("should iterate all keys", async function () {
+      const bigMap = createBigMap (testMap);
+      assert.sameMembers([...bigMap.keys()], [...testMap.keys()]);
+    });
+  });
+
+  describe("values", function () {
+    it("should get all values", async function () {
+      const bigMap = createBigMap (testMap);
+      assert.sameMembers([...bigMap.values()], [...testMap.values()]);
+    });
+  });
+
+  describe("entries", function () {
+    it("should get all values", async function () {
+      const bigMap = createBigMap (testMap);
+      const actualMap  = new Map([...bigMap.entries()]);
+      assert.deepEqual(actualMap, testMap);
+    });
+  });
+
+  describe("iterator", function () {
+    it("should get all values", async function () {
+      const bigMap = createBigMap (testMap);
+
+      const actualMap  = new Map();
+      for (const entry of bigMap) {
+        actualMap.set(entry[0], entry[1]);
+      }
+      assert.deepEqual(actualMap, testMap);
+    });
+  });
+
+  describe("toStringTag", function () {
+    it("should return type name", async function () {
+      const typeName = Object.prototype.toString.call(new BigMap <string> ());
+      assert.equal(typeName, "[object BigMap]");
+    });
+  });
+
+  describe("has", function () {
+    it("should return true when value was set", async function () {
+      const bigMap = new BigMap <string> ();
+      const key = "0x123f";
+      bigMap.set(key, "12134");
+      assert.isTrue (bigMap.has(key));
+    });
+  });
+
+  describe("set", function () {
+    it("should set when key has submap", async function () {
+      const bigMap = new BigMap <string> ();
+      assert.doesNotThrow(() => bigMap.set("0x13", "12134"));
+      assert.equal(bigMap.size, 1);
+    });
+
+    it("should throw when key has no submap", async function () {
+      const bigMap = new BigMap <string> ();
+      assert.throw(() => bigMap.set("g", "12134"), "Tried to set g, but that key has no submap");
+    });
+  });
+});
diff --git a/packages/transformer/src/test/standalone/IModelTransformer.test.ts b/packages/transformer/src/test/standalone/IModelTransformer.test.ts
index 10913357..65341aba 100644
--- a/packages/transformer/src/test/standalone/IModelTransformer.test.ts
+++ b/packages/transformer/src/test/standalone/IModelTransformer.test.ts
@@ -19,7 +19,7 @@ import {
 import * as coreBackendPkgJson from "@itwin/core-backend/package.json";
 import * as ECSchemaMetaData from "@itwin/ecschema-metadata";
 import * as TestUtils from "../TestUtils";
-import { DbResult, Guid, Id64, Id64String, Logger, LogLevel, OpenMode } from "@itwin/core-bentley";
+import { DbResult, Guid, Id64, Id64String, Logger, LoggingMetaData, LogLevel, OpenMode } from "@itwin/core-bentley";
 import {
   AxisAlignedBox3d, BriefcaseIdValue, Code, CodeScopeSpec, CodeSpec, ColorDef, CreateIModelProps, DefinitionElementProps, ElementAspectProps, ElementProps,
   ExternalSourceAspectProps, GeometricElement2dProps, ImageSourceFormat, IModel, IModelError, InformationPartitionElementProps, ModelProps, PhysicalElementProps, Placement3d, ProfileOptions, QueryRowFormat, RelatedElement, RelationshipProps, RepositoryLinkProps,
@@ -71,6 +71,9 @@ describe("IModelTransformer", () => {
     if (!IModelJsFs.existsSync(outputDir)) {
       IModelJsFs.mkdirSync(outputDir);
     }
+  });
+
+  beforeEach(async () => {
     // initialize logging
     if (process.env.LOG_TRANSFORMER_IN_TESTS) {
       Logger.initializeToConsole();
@@ -642,6 +645,55 @@ describe("IModelTransformer", () => {
     iModelShared.close();
   });
 
+  it("should log unresolved references", async () => {
+    const iModelShared: SnapshotDb = IModelTransformerTestUtils.createSharedIModel(outputDir, ["A", "B"]);
+    const iModelA: SnapshotDb = IModelTransformerTestUtils.createTeamIModel(outputDir, "A", Point3d.create(0, 0, 0), ColorDef.green);
+    IModelTransformerTestUtils.assertTeamIModelContents(iModelA, "A");
+    const iModelExporterA = new IModelExporter(iModelA);
+
+    // Exclude element
+    const excludedId = iModelA.elements.queryElementIdByCode(Subject.createCode(iModelA, IModel.rootSubjectId, "Context"));
+    assert.isDefined (excludedId);
+    iModelExporterA.excludeElement(excludedId!);
+
+    const subjectId: Id64String = IModelTransformerTestUtils.querySubjectId(iModelShared, "A");
+    const transformerA2S = new IModelTransformer(iModelExporterA, iModelShared, { targetScopeElementId: subjectId });
+    transformerA2S.context.remapElement(IModel.rootSubjectId, subjectId);
+
+    // Configure logger to capture warning message about unresolved references
+    const messageStart = "The following elements were never fully resolved:\n";
+    const messageEnd = "\nThis indicates that either some references were excluded from the transformation\nor the source has dangling references.";
+
+    let unresolvedElementMessage: string | undefined;
+    const logWarning = (_category: string, message: string, _metaData: LoggingMetaData) => {
+      if (message.startsWith (messageStart)) {
+        unresolvedElementMessage = message;
+      }
+    };
+    Logger.initialize(undefined, logWarning);
+    Logger.setLevelDefault(LogLevel.Warning);
+
+    // Act
+    await transformerA2S.processAll();
+
+    // Collect expected ids
+    const result = iModelA.queryEntityIds({ from: "BisCore.Element",
+      where: "Model.Id = :rootId AND ECInstanceId NOT IN (:rootId, :excludedId)",
+      bindings: {rootId: IModel.rootSubjectId, excludedId } });
+    const expectedIds = [...result].map ((x) => `e${x}`);
+
+    // Collect actual ids
+    assert.isDefined(unresolvedElementMessage);
+    const actualIds = unresolvedElementMessage!.split(messageStart)[1].split(messageEnd)[0].split(",");
+
+    // Assert
+    assert.sameMembers(actualIds, expectedIds);
+
+    transformerA2S.dispose();
+    iModelA.close();
+    iModelShared.close();
+  });
+
   it("should detect conflicting provenance scopes", async () => {
     const sourceDb1 = IModelTransformerTestUtils.createTeamIModel(outputDir, "S1", Point3d.create(0, 0, 0), ColorDef.green);
     const sourceDb2 = IModelTransformerTestUtils.createTeamIModel(outputDir, "S2", Point3d.create(0, 10, 0), ColorDef.blue);