GeoPandas within() not returning any records

Question

I'm trying to create a layer of all cities with 100km of the coast of Australia. The following is returning coastCities as an empty DataFrame.

I have performed the same operations in ArcGIS and it works fine, returning 135 records. I suspect it's the within() part that I'm getting wrong but I can't work out what's going on. cities is a GeoDataFrame with several thousand records with point geometry and Australia is a GeoDataFrame with 1 MultiPolygon row. The buffering works fine, I plotted that and it's good.

import geopandas as gpd
import matplotlib.pyplot as plt

Australia = gpd.read_file('australia_Main_Dissolve.shp')
cities = gpd.read_file('ne_10m_populated_places.shp')

AusCoast_buffer = Australia.boundary.buffer(100000)

coastCities = cities.loc[cities.within(AusCoast_buffer)]

Edit: corrected description of variables

Answer 1

I found the answer in this post.

By changing the lines after the file reads to

AusCoast_buffer = gpd.GeoDataFrame(Australia.boundary.buffer(100000), columns=['geometry'])

AusCoast_buffer_u = AusCoast_buffer.geometry.unary_union

AusCoast_cities = cities.loc[cities.within(AusCoast_buffer_u)]

I get what I want. It selects the correct records in cities. However AusCoast_buffer_u is of type shapely.geometry.multipolygon.MultiPolygon and if I try to convert it to a GeoDataFrame (so that it can be easily plotted) before doing the selection then within() goes back to not selecting any cities records. I really don't understand what is happening here, any explanation would be very welcome.