Carbon sink tax bill 'seriously flawed'

There were extraordinary scenes in the Senate last night as the Greens combined with the Nationals and the Liberal Senator Bill Heffernan to attack part of a tax bill.

It was originally drafted by the previous Coalition government and has been reintroduced by Labor.

It gives a full tax deduction for the cost of planting forests as carbon sinks.

But Senator Heffernan, the Nationals and the Greens argued passionately in the Senate last night that it is flawed.

Senator Heffernan says the bill does not stop the trees being cut down or allowed to die within a year.

"There is absolutely no way in this legislation that you can prevent me from planting these trees and the year after I collect the tax deduction - me ploughing them in," he said.

"It is a national disgrace if we allow this Parliament to pass this legislation in the full knowledge that it is completely and fundamentally flawed."

Greens Senator Bob Brown agrees the bill is seriously flawed.

Nationals Senator Barnaby Jocye has called it a load of rubbish.

"Its very rare - to be honest - I would agree with the Greens but tonight's going to be one of those nights," Senator Joyce said.

But Government Senator Stephen Conroy says the bill was initially brought in by the previous Government.

"To be muttering up the back about what an evil plot this is - its your bill," he said.

The senators do not deny that they are negotiating on ways to to fix the problems they have complained about.

The Coalition is understood to be discussing technical aspects of how to do that.