Thank you Jim for the tip regarding entrance reducer.
Blackforest beekeeper,
I agree that there seem to always be some food for the bees in Australia (but my knowledge and experience are very limited!) In my garden alone, there are lots of flowers for them, and I am in a semi-rural / very green suburban area - so no doubt there are some sources of nectar and pollen (although we have had a terribly hot and dry season). I do see them come home quite loaded, however the population is not growing. And beekeepers here definitely use some sugar syrup at times.
Blackforest Beekeeper, what else do you suggest then? I don't think I could even get another package of bees at this time of year (end of Feb is the end of Summer here). What else could be wrong with them? They do look healthy, the brood pattern is nice and compact (but small) and there is one or two heavy frames full of honey. But no expansion...
Thanks!