Whether its the lot owner, the owners corporation or the builder, doing nothing is not an option. Non action just causes more damage.
If the builder is being stubborn and it’s common property, the OC should get the repairs done , or some temporary fix so further damage is not done.
I assume you have reports from qualified people identifying the problem and the solutions. You’ll need these in any case in fair trading.
The courts will not leave you out of pocket if you take reasonable steps to minimise losses.
One other item to remember is that by the OC not taking action to minimise the lot owners losses, the OC will be liable for these losses.