What is personally more concerning in my humble observation is the sheer volume of apartments and units (12-packs and 18-packs mostly - with little to no parking!) That are popping up everywhere in Sydney'spoor-access suburbs in the 10km, 20km, 30km, 40km suburbs. A lot of these areas are family 'heartland' and developers cramming way too many small units into these areas is a red flag for me. So I point the question back at Sydney and ask "who is going to live in this new block of 1-bed, 0-parking units that is built 27km from the city (and is 3+ km from the nearest train station, and takes 3x connecting buses to get to cbd via public transport etc.).
Too me, that presents as a great risk for metro Sydney. Like I said, they are doing the right thing and building 'up' in the areas where it is most needed (I.e. inner ring, and then on train nodes in the suburban network), that doesn't concern me. It's the alarming volume of units being built in far flung, difficult to access suburbs, on teeny tiny skerricks of land, that concerns me. In this sense, I think pockets of Sydney will see an oversupply. I know I'll cop a lot of flack for suggesting that ANY of Sydney +50km could have a risk of oversupply, but its about building the right stock type in the right areas; something that I'm not sure is being done.
Brisbane is (somewhat ) more sensible at the moment but faces different challenges. I'd argue that units are being built in mostly the right places for the right demo's, but at too big a scale based on population growth predictions. The other dynamic in Brisbane is that (again, in my humble opinion only), I've observed different housing stock behaviours. People there appear to be more adverse to apartment living; they still value back yards, more space, pet-friendly etc. much more than most top-dollar-job-obsessed folks in Sydney, and I think they still value houses over units in many parts.