diff --git a/app/models/broadcast_message.py b/app/models/broadcast_message.py
index d6f735837..a90a6a821 100644
--- a/app/models/broadcast_message.py
+++ b/app/models/broadcast_message.py
@@ -62,9 +62,7 @@ class BroadcastMessage(JSONModel):
 
     @property
     def areas(self):
-        return broadcast_area_libraries.get_areas(
-            *self._dict['areas']
-        )
+        return self.get_areas(areas=self._dict['areas'])
 
     @property
     def initial_area_names(self):
@@ -82,14 +80,7 @@ class BroadcastMessage(JSONModel):
 
     @cached_property
     def simple_polygons(self):
-        polygons = Polygons(
-            list(itertools.chain(*(
-                area.simple_polygons for area in self.areas
-            )))
-        )
-        # If we’ve added multiple areas then we need to re-simplify the
-        # combined shapes to keep the point count down
-        return polygons.smooth.simplify if len(self.areas) > 1 else polygons
+        return self.get_simple_polygons(areas=self.areas)
 
     @property
     def template(self):
@@ -122,16 +113,35 @@ class BroadcastMessage(JSONModel):
     def cancelled_by(self):
         return User.from_id(self.cancelled_by_id)
 
+    def get_areas(self, areas):
+        return broadcast_area_libraries.get_areas(
+            *areas
+        )
+
+    def get_simple_polygons(self, areas):
+        polygons = Polygons(
+            list(itertools.chain(*(
+                area.simple_polygons for area in areas
+            )))
+        )
+        # If we’ve added multiple areas then we need to re-simplify the
+        # combined shapes to keep the point count down
+        return polygons.smooth.simplify if len(areas) > 1 else polygons
+
     def add_areas(self, *new_areas):
-        self._update(areas=list(OrderedSet(
+        areas = list(OrderedSet(
             self._dict['areas'] + list(new_areas)
-        )))
+        ))
+        simple_polygons = self.get_simple_polygons(areas=self.get_areas(areas=areas))
+        self._update(areas=areas, simple_polygons=simple_polygons.as_coordinate_pairs_lat_long)
 
     def remove_area(self, area_to_remove):
-        self._update(areas=[
+        areas = [
             area for area in self._dict['areas']
             if area != area_to_remove
-        ])
+        ]
+        simple_polygons = self.get_simple_polygons(areas=self.get_areas(areas=areas))
+        self._update(areas=areas, simple_polygons=simple_polygons.as_coordinate_pairs_lat_long)
 
     def _set_status_to(self, status):
         broadcast_message_api_client.update_broadcast_message_status(
diff --git a/tests/app/main/views/test_broadcast.py b/tests/app/main/views/test_broadcast.py
index c73bca1dd..37b8d7ce6 100644
--- a/tests/app/main/views/test_broadcast.py
+++ b/tests/app/main/views/test_broadcast.py
@@ -1,5 +1,6 @@
 import json
 import uuid
+from collections import namedtuple
 from functools import partial
 
 import pytest
@@ -507,8 +508,14 @@ def test_add_broadcast_area(
     mock_get_draft_broadcast_message,
     mock_update_broadcast_message,
     fake_uuid,
+    mocker
 ):
     service_one['permissions'] += ['broadcast']
+    polygon_class = namedtuple("polygon_class", ["as_coordinate_pairs_lat_long"])
+    coordinates = [[50.1, 0.1], [50.2, 0.2], [50.3, 0.2]]
+    polygons = polygon_class(as_coordinate_pairs_lat_long=coordinates)
+    mocker.patch('app.models.broadcast_message.BroadcastMessage.get_simple_polygons', return_value=polygons)
+
     client_request.post(
         '.choose_broadcast_area',
         service_id=SERVICE_ONE_ID,
@@ -522,7 +529,7 @@ def test_add_broadcast_area(
         service_id=SERVICE_ONE_ID,
         broadcast_message_id=fake_uuid,
         data={
-            'areas': ['ctry19-E92000001', 'ctry19-S92000003', 'ctry19-W92000004']
+            'areas': ['ctry19-E92000001', 'ctry19-S92000003', 'ctry19-W92000004'], 'simple_polygons': coordinates
         },
     )
 
@@ -555,8 +562,14 @@ def test_add_broadcast_sub_area(
     fake_uuid,
     post_data,
     expected_selected,
+    mocker
 ):
     service_one['permissions'] += ['broadcast']
+    polygon_class = namedtuple("polygon_class", ["as_coordinate_pairs_lat_long"])
+    coordinates = [[50.1, 0.1], [50.2, 0.2], [50.3, 0.2]]
+    polygons = polygon_class(as_coordinate_pairs_lat_long=coordinates)
+    mocker.patch('app.models.broadcast_message.BroadcastMessage.get_simple_polygons', return_value=polygons)
+
     client_request.post(
         '.choose_broadcast_sub_area',
         service_id=SERVICE_ONE_ID,
@@ -569,6 +582,7 @@ def test_add_broadcast_sub_area(
         service_id=SERVICE_ONE_ID,
         broadcast_message_id=fake_uuid,
         data={
+            'simple_polygons': coordinates,
             'areas': [
                 # These two areas are on the broadcast already
                 'ctry19-E92000001',
@@ -584,8 +598,14 @@ def test_remove_broadcast_area_page(
     mock_get_draft_broadcast_message,
     mock_update_broadcast_message,
     fake_uuid,
+    mocker,
 ):
     service_one['permissions'] += ['broadcast']
+    polygon_class = namedtuple("polygon_class", ["as_coordinate_pairs_lat_long"])
+    coordinates = [[50.1, 0.1], [50.2, 0.2], [50.3, 0.2]]
+    polygons = polygon_class(as_coordinate_pairs_lat_long=coordinates)
+    mocker.patch('app.models.broadcast_message.BroadcastMessage.get_simple_polygons', return_value=polygons)
+
     client_request.get(
         '.remove_broadcast_area',
         service_id=SERVICE_ONE_ID,
@@ -602,6 +622,7 @@ def test_remove_broadcast_area_page(
         service_id=SERVICE_ONE_ID,
         broadcast_message_id=fake_uuid,
         data={
+            'simple_polygons': coordinates,
             'areas': ['ctry19-S92000003']
         },
     )