Begin photo-documenting everything regarding the leak. Call and notify the builder so they can come take a look. Send a follow-up/recap of your call through to their email to begin a paper trail of your notification of the leak to them. Try to keep as much correspondence in writing (email) as possible.

See what the builder says, give them a chance to make it right. If they are a legitimate builder they will want to take a look and make it right if they are at fault. Bring up your concerns (leak causing damage to timber floorboards and structural beams, mould, etc). See what they propose. It also may be best for them to involve their insurance if they are liable as repairs may be costly. It's also important for you to document everything as not all damages are immediately visible (timber floorboards may swell or rise after prolonged exposure to leak, etc).

If the builder are dismissive or combatant you can enlist an engineer to come out to your house and do an expert report on the alleged damages or faulty workmanship.

If you are in Victoria you can take the builder to VCAT. Submit the expert report along with a record of your correspondence. Though the VCAT process can be lengthy they will make a determination on the case based on evidence.

The builder should have also taken out warranty insurance on the job if it was a larger renovation. The warranty insurance differs by state but would generally kick in if there are defects and the builder is unable to fix them (dead, missing, insolvent, or unable to comply with a statutory order in Victoria).