TONY ABBOTT has described the carbon tax as ''the longest political suicide note in Australian history'' and a ''completely pointless exercise'' because it would allow Australian companies to buy some greenhouse gas emission reductions overseas.

But the Australian Industry Greenhouse Network - which represents mining and manufacturing industries - said it agreed with the federal Treasury that Mr Abbott's plan of achieving 5 per cent emission reductions domestically would at least double the cost, whether done with a carbon price or through his proposed ''direct action''.

"Completely pointless exercise" ... Tony Abbott on the Government's carbon tax. Credit:Alex Ellinghausen

The industry network and member groups such as the Business Council of Australia want Labor to allow business to buy more permits overseas.

Mr Abbott told Parliament that Labor's plan was a ''pointless exercise'' because allowing companies to buy some permits overseas meant that by 2020 Australia's domestic emissions would be higher, not lower, than they are now.