diff --git a/app/notify_client/organisations_api_client.py b/app/notify_client/organisations_api_client.py
index 1d18ebaa8..ecec12359 100644
--- a/app/notify_client/organisations_api_client.py
+++ b/app/notify_client/organisations_api_client.py
@@ -10,8 +10,9 @@ class OrganisationsClient(NotifyAdminAPIClient):
     def get_organisations(self):
         return self.get(url='/organisations')
 
+    @cache.set('domains')
     def get_domains(self):
-        return set(chain.from_iterable(
+        return list(chain.from_iterable(
             organisation['domains']
             for organisation in self.get_organisations()
         ))
@@ -35,6 +36,7 @@ class OrganisationsClient(NotifyAdminAPIClient):
         }
         return self.post(url="/organisations", data=data)
 
+    @cache.delete('domains')
     def update_organisation(self, org_id, **kwargs):
         return self.post(url="/organisations/{}".format(org_id), data=kwargs)
 
diff --git a/tests/app/notify_client/test_organisation_client.py b/tests/app/notify_client/test_organisation_client.py
new file mode 100644
index 000000000..7839cbdad
--- /dev/null
+++ b/tests/app/notify_client/test_organisation_client.py
@@ -0,0 +1,94 @@
+from unittest.mock import call
+
+import pytest
+
+from app import organisations_client
+
+
+@pytest.mark.parametrize(
+    (
+        'expected_cache_get_calls,'
+        'cache_value,'
+        'expected_api_calls,'
+        'expected_cache_set_calls,'
+        'expected_return_value,'
+    ),
+    [
+        (
+            [
+                call('domains')
+            ],
+            b"""
+                [
+                    {"domains": ["a", "b", "c"]},
+                    {"domains": ["c", "d", "e"]}
+                ]
+            """,
+            [],
+            [],
+            ['a', 'b', 'c', 'd', 'e'],
+        ),
+        (
+            [
+                call('domains')
+            ],
+            None,
+            [
+                call(url='/organisations')
+            ],
+            [
+                call(
+                    'domains',
+                    '["x", "y", "z"]',
+                    ex=604800
+                )
+            ],
+            'from api',
+        ),
+    ]
+)
+def test_returns_value_from_cache(
+    app_,
+    mocker,
+    expected_cache_get_calls,
+    cache_value,
+    expected_return_value,
+    expected_api_calls,
+    expected_cache_set_calls,
+):
+
+    mock_redis_get = mocker.patch(
+        'app.extensions.RedisClient.get',
+        return_value=cache_value,
+    )
+    mock_api_get = mocker.patch(
+        'app.notify_client.NotifyAdminAPIClient.get',
+        return_value=[
+            {'domains': ['x', 'y', 'z']}
+        ],
+    )
+    mock_redis_set = mocker.patch(
+        'app.extensions.RedisClient.set',
+    )
+
+    organisations_client.get_domains()
+
+    assert mock_redis_get.call_args_list == expected_cache_get_calls
+    assert mock_api_get.call_args_list == expected_api_calls
+    assert mock_redis_set.call_args_list == expected_cache_set_calls
+
+
+def test_deletes_domain_cache(
+    app_,
+    mock_get_user,
+    mocker,
+    fake_uuid,
+):
+    mocker.patch('app.notify_client.current_user', id='1')
+    mock_redis_delete = mocker.patch('app.extensions.RedisClient.delete')
+    mock_request = mocker.patch('notifications_python_client.base.BaseAPIClient.request')
+
+    organisations_client.update_organisation(fake_uuid, foo='bar')
+
+    assert call('domains') in mock_redis_delete.call_args_list
+    assert len(mock_request.call_args_list) == 1