Leaky bathroom repaired but still leaking

I recently purchased my first home but had a building report prepared before purchasing. The report identified a leak in the ensuite so a condition was included in the Sales and Purchase agreement to have the issue "properly remedied and made good, including the repair and repaiting of the damaged wall surface." The repair was completed prior to us moving in and included a 3 year warranty on the works completed. Settlement on the property was March 27th 2025.

I recently had moisture testing completed in the ensuite and the readings came back really high, suggesting that the issue hasnt been resolved. The leak is also in the same area as was originally identified in the building report (inside the wall / behind the tiles next to the shower). We have been in contact with the original plumber, builder and tiler to try and identify the cause of the leak, which they havent been able to do yet (because investigating further will likely involve removing tiles, etc). We're at the stage now where the tradies are blaming the leak on a different issue that wasnt part of their original scope of works and repair. Accordingly, they're saying another repair isnt covered under the warranty.

The repair was done only 4 months ago so it seems unlikely that a new leak has come about in that time.

Who is liable for the repair in this situation? The bathroom repair company, the previous owner or my partner and I? Were potentially looking at a 20k bill to have the leak repaired and bathroom replaced.

Really appreciate any advice here.