During the first six months the burden of proof works for you. Secure the defect now.
If the defect in the property appears within six months of handover, section 924 ABGB presumes it already existed at handover. The seller would have to prove otherwise. You can demand repair regardless of fault; only if repair fails do price reduction or rescission come into play (section 932 ABGB).
Document the defect with photos and dates and request repair in writing. At the same time, check whether the purchase contract contains an exclusion clause and whether it covers this defect at all.