The Australian Bureau of Statistics' spend on contractors is coming at a high cost as the agency admits it pays them up to double the rate compared to internal staff.

Agency officials have told a parliamentary inquiry that IT contractors cost double the salary of public servants doing the same work, while non-IT labour hire workers can receive up to 50 per cent more.

The admission provoked a backlash from Labor as its Finance spokesman Jim Chalmers and inquiry committee member Julian Hill called it a "smoking gun" exposing the Coalition government's waste in using labour hire firms.

'Smoking gun': Labor Finance spokesman Jim Chalmers has slammed the Coalition government's waste in using labour hire firms. Credit:Alex Ellinghausen

"This latest evidence is the first time the public has been given a straight answer about how much of their money is being wasted, and it raises serious concerns about the Liberals’ spending blow outs," they said in a statement.