I think it’s in the Home Building Act 1989 Schedule 1 definitions of residential building work, specialist work, and plumbing and drainage work and in the Home Building Regulation 5 Thresholds for contract requirements. I started investigating. Still not sure that waterproofing under $5000 is exempt though, as it still may be specialist work.
Not easy or practical to work out what’s lawful and how to maintain your building on a budget. But fun if you enjoy that kind of thing.