Check the Licence First
In NSW, plumbers must hold a current plumber's licence issued by NSW Fair Trading. You can verify any licence on the NSW Fair Trading public register at nsw.gov.au. A plumber who can't or won't give you their licence number is unlicensed — full stop. Unlicensed plumbing work is illegal in NSW and creates serious liability for you as the property owner.
For gas work, check they also hold a gas fitting licence. Not all plumbers are also licensed gas fitters — if the job involves gas, confirm both licences.