Yes. If you are on Centrelink benefits, any extra income you earn needs to be declared—including cash in hand jobs. Centrelink is known to investigate reports into undeclared wages.
The Welfare Rights Centre can give you advice on problems with Centrelink. They are a free and independent legal centre providing information, advice and advocacy.
Phone: (02) 9211 5300 or Free Call: 1800 226 028