First off - I discovered that Toongabbie - the suburb I'm interested in, straddles two SA4s, so I extended my filter to include both the Blacktown and Parramatta SA4s:
SA2_map.objects.tracts.geometries = SA2_map.objects.tracts.geometries.filter(d => d.properties.SA4_NAME16 == "Blacktown" || d.properties.SA4_NAME16 =="Parramatta")
I first drew the map using SA4 simplified topojson )see day five to see how these files were made , gave each one a different grey fill, and a thicker stroke. With the SA2s, I gave a thinner stroke, and filled the SA4s that capture Toongabbie the same
const SA2_colour = ({
"Toongabbie - Constitution Hill": '#FF96AD',
"Seven Hills - Toongabbie": '#FF96AD',
})
some more code...
...
.attr("fill", (d) => SA2_colour[d.properties.SA2_NAME16] || 'none')
And more as a reminder to myself...all of this is on github here