The Australian Greens say Palmer United Party senators should abstain from voting on the carbon tax repeal bills because of a conflict of interest.
PUP leader and businessman Clive Palmer abstained from the vote on the bills in the lower house because of a possible conflict of interest over his mining firms, which are impacted by the tax.
His party's senators Glenn Lazarus from Queensland and Tasmania's Jacqui Lambie, and possibly a third in Western Australia after this weekend's election, will have a crucial vote in the upper house.
Mr Palmer says the carbon tax repeal should be made retrospective, which would financially benefit his companies.
"Surely his two senators cannot vote to personally and directly financially advantage the leader of the political party to the tune of multi-millions," Greens leader Senator Christine Milne told the National Press Club in Canberra on Tuesday.
"If it's good enough for him to abstain in the House, then it is good enough for them to abstain in the Senate."
She said if this did not occur it would set a precedent for corporations to use their profits to elect senators and MPs to change laws that adversely affect them.
"Forget the national interest - it would be democracy for sale," Senator Milne said.
One of Mr Palmer's companies Queensland Nickel reportedly owes more than $8.4 million in carbon tax liabilities with a potential penalty of more than $35 million if it is not paid on time.
Mr Palmer told reporters on Tuesday he understood the company had paid its bill on time.
A spokesperson for the Clean Energy Regulator told AAP on Tuesday the agency had been advised by Queensland Nickel that the company had "made a payment towards the outstanding debt" to the CER.
"However, at this stage we cannot confirm that we have received any payment from Queensland Nickel."
