Two prominent Liberal MPs billed taxpayers thousands in expenses on a day they travelled to Sydney to toast the cause of ending wasteful government spending at a function for a new activist group.

Senators Eric Abetz and Cory Bernardi claimed more than $2000 in expenses for travel to Sydney when they attended the 2012 local launch of the Australian Taxpayers' Alliance.

The ATA describes itself as an "revolutionary grassroots activist body" devoted to fighting "crippling levels" of government waste.

The Senators claimed for flights and about $800 in "official business" allowances on the day of the function.