In NSW, a licence is required for residential building work worth more than $5,000 including GST in labour and materials. Specialist work such as electrical, plumbing, drainage, gasfitting and air conditioning needs a licence whatever the cost (NSW Government).
You can look up any tradie’s licence, the categories of work it covers, its expiry date and any suspensions on Verify NSW. It takes under a minute, so do it before you sign anything.