diff --git a/services/ml_training_data/pyproject.toml b/services/ml_training_data/pyproject.toml
index 232d2276..4519fd93 100644
--- a/services/ml_training_data/pyproject.toml
+++ b/services/ml_training_data/pyproject.toml
@@ -6,6 +6,7 @@ requires-python = ">=3.11"
 dependencies = [
     "pandas>=2.0",
     "pandas-stubs",
+    "ijson>=3.2",
 ]
 
 [build-system]
diff --git a/services/ml_training_data/src/ml_training_data/bulk_zip_reader.py b/services/ml_training_data/src/ml_training_data/bulk_zip_reader.py
new file mode 100644
index 00000000..0790d172
--- /dev/null
+++ b/services/ml_training_data/src/ml_training_data/bulk_zip_reader.py
@@ -0,0 +1,46 @@
+"""Stream EPC certificates from the gov bulk JSON ZIP.
+
+The bulk ZIP from /api/files/domestic/json is ZIP64, ~15 GB, with one JSON entry
+per inspection year (certificates-YYYY.json). Each entry is a JSON array of
+certificate dicts. The reader streams entries via ijson so 5GB+ yearly files
+never have to fit in memory.
+"""
+
+import zipfile
+from collections.abc import Iterator
+from typing import Any, cast
+
+import ijson  # type: ignore[import-untyped]
+
+from ml_training_data.storage import Storage
+
+
+class BulkZipReader:
+    def __init__(self, storage: Storage, zip_key: str) -> None:
+        self._storage = storage
+        self._zip_key = zip_key
+
+    def list_entries(self) -> list[str]:
+        with zipfile.ZipFile(self._storage.open_read(self._zip_key)) as zf:
+            return zf.namelist()
+
+    def iter_certificates(self, entry: str) -> Iterator[dict[str, Any]]:
+        with zipfile.ZipFile(self._storage.open_read(self._zip_key)) as zf:
+            with zf.open(entry) as f:
+                for item in ijson.items(f, "item"):
+                    yield cast(dict[str, Any], item)
+
+    def iter_certificates_filtered(
+        self, certificate_numbers: set[str]
+    ) -> Iterator[dict[str, Any]]:
+        remaining = set(certificate_numbers)
+        for entry in self.list_entries():
+            if not remaining:
+                return
+            for cert in self.iter_certificates(entry):
+                cert_num = cert.get("certificate_number")
+                if cert_num in remaining:
+                    remaining.discard(cert_num)
+                    yield cert
+                    if not remaining:
+                        return
diff --git a/services/ml_training_data/src/ml_training_data/storage.py b/services/ml_training_data/src/ml_training_data/storage.py
index c5c76490..8e7e3994 100644
--- a/services/ml_training_data/src/ml_training_data/storage.py
+++ b/services/ml_training_data/src/ml_training_data/storage.py
@@ -7,7 +7,7 @@ the swap is a constructor change, not a callsite rewrite.
 
 from collections.abc import Iterator
 from pathlib import Path
-from typing import Protocol
+from typing import IO, Protocol
 
 
 class Storage(Protocol):
@@ -15,6 +15,7 @@ class Storage(Protocol):
     def read_bytes(self, key: str) -> bytes: ...
     def exists(self, key: str) -> bool: ...
     def iter_keys(self, prefix: str = "") -> Iterator[str]: ...
+    def open_read(self, key: str) -> IO[bytes]: ...
 
 
 class LocalStorage:
@@ -35,6 +36,9 @@ class LocalStorage:
     def exists(self, key: str) -> bool:
         return self._path(key).exists()
 
+    def open_read(self, key: str) -> IO[bytes]:
+        return self._path(key).open("rb")
+
     def iter_keys(self, prefix: str = "") -> Iterator[str]:
         if not self._root.exists():
             return
diff --git a/services/ml_training_data/tests/unit/test_bulk_zip_reader.py b/services/ml_training_data/tests/unit/test_bulk_zip_reader.py
new file mode 100644
index 00000000..a7ac0409
--- /dev/null
+++ b/services/ml_training_data/tests/unit/test_bulk_zip_reader.py
@@ -0,0 +1,89 @@
+"""Tests for BulkZipReader — stream EPC certificates from the gov bulk JSON ZIP.
+
+The real bulk ZIP is ZIP64, ~15 GB, with one JSON entry per inspection year
+(certificates-YYYY.json). Each entry is a JSON array of certificate dicts.
+We test against small synthetic ZIPs of the same shape.
+"""
+
+import io
+import json
+import zipfile
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+from ml_training_data.bulk_zip_reader import BulkZipReader
+from ml_training_data.storage import LocalStorage
+
+
+def _write_zip(storage: LocalStorage, key: str, entries: dict[str, Any]) -> None:
+    buf = io.BytesIO()
+    with zipfile.ZipFile(buf, "w", zipfile.ZIP_DEFLATED) as zf:
+        for entry_name, payload in entries.items():
+            zf.writestr(entry_name, json.dumps(payload))
+    storage.write_bytes(key, buf.getvalue())
+
+
+def test_list_entries_returns_zip_member_names_in_archive_order(tmp_path: Path) -> None:
+    # Arrange
+    storage = LocalStorage(root=tmp_path)
+    _write_zip(
+        storage,
+        "bulk.zip",
+        {"certificates-2024.json": [], "certificates-2025.json": []},
+    )
+    reader = BulkZipReader(storage=storage, zip_key="bulk.zip")
+
+    # Act
+    entries = reader.list_entries()
+
+    # Assert
+    assert entries == ["certificates-2024.json", "certificates-2025.json"]
+
+
+def test_iter_certificates_yields_every_cert_in_entry(tmp_path: Path) -> None:
+    # Arrange
+    storage = LocalStorage(root=tmp_path)
+    certs = [
+        {"certificate_number": "CN-0001", "postcode": "AA1 1AA"},
+        {"certificate_number": "CN-0002", "postcode": "BB2 2BB"},
+        {"certificate_number": "CN-0003", "postcode": "CC3 3CC"},
+    ]
+    _write_zip(storage, "bulk.zip", {"certificates-2025.json": certs})
+    reader = BulkZipReader(storage=storage, zip_key="bulk.zip")
+
+    # Act
+    out = list(reader.iter_certificates("certificates-2025.json"))
+
+    # Assert
+    assert out == certs
+
+
+def test_iter_certificates_filtered_yields_only_requested_across_entries(tmp_path: Path) -> None:
+    # Arrange
+    storage = LocalStorage(root=tmp_path)
+    _write_zip(
+        storage,
+        "bulk.zip",
+        {
+            "certificates-2024.json": [
+                {"certificate_number": "A-1", "postcode": "AA1"},
+                {"certificate_number": "A-2", "postcode": "AA2"},
+            ],
+            "certificates-2025.json": [
+                {"certificate_number": "B-1", "postcode": "BB1"},
+                {"certificate_number": "B-2", "postcode": "BB2"},
+            ],
+        },
+    )
+    reader = BulkZipReader(storage=storage, zip_key="bulk.zip")
+
+    # Act
+    out = sorted(
+        reader.iter_certificates_filtered({"A-1", "B-2", "MISSING-9"}),
+        key=lambda c: c["certificate_number"],
+    )
+
+    # Assert
+    assert [c["certificate_number"] for c in out] == ["A-1", "B-2"]
diff --git a/services/ml_training_data/tests/unit/test_storage.py b/services/ml_training_data/tests/unit/test_storage.py
index 0ab08051..c254c2c9 100644
--- a/services/ml_training_data/tests/unit/test_storage.py
+++ b/services/ml_training_data/tests/unit/test_storage.py
@@ -74,3 +74,17 @@ def test_read_bytes_raises_filenotfound_for_missing_key(tmp_path: Path) -> None:
     # Act / Assert
     with pytest.raises(FileNotFoundError):
         storage.read_bytes("nope.bin")
+
+
+def test_open_read_returns_seekable_binary_stream(tmp_path: Path) -> None:
+    # Arrange
+    storage = LocalStorage(root=tmp_path)
+    storage.write_bytes("big.bin", b"abcdefghij")
+
+    # Act
+    with storage.open_read("big.bin") as f:
+        f.seek(4)
+        chunk = f.read(3)
+
+    # Assert
+    assert chunk == b"efg"