Not sure if it's entirely reliable, but this article indicates Sydney is not oversupplied with residential units, but Brisbane very soon will be.
The picture was different in Brisbane, where decade-low levels of
migration into southeast Queensland and the recent state budget boost
for first home buyers were weakening the rental market. Rents of units
fell to $375 from March's record $380 and houses slipped to $400 from
$410. While prices are still up, or equal to, their level of a year ago,
vacancy rates for both dwelling types have increased, suggesting rents
will fall further - especially given the large numbers of new apartments
due to settle in the Queensland capital, Dr Wilson said.
"The signs are all there for more supply than demand," he said. "It means downward pressure on rents in Brisbane."