AAP

Federal Treasury says the government will have to think about boosting household assistance under its carbon price package if the world emissions price is higher than forecast in 2015.

But Treasury boss Martin Parkinson says on the flip side, if the international price is lower than expected when emissions trading begins, households will be better off because compensation won't be cut.

Independent senator Nick Xenophon on Thursday quizzed Treasury officials on the impact of a world price that was substantially different than modelled.

He pointed out that Bloomberg New Energy Finance, relying on 200 analysts around the world, has forecast the international carbon price will be $16-a-tonne in 2015.

Treasury has forecast it will be $29.

Senator Xenophon said that could lead to a revenue shortfall of $4 billion.

Treasury official Luise McCulloch argued that wasn't exactly true because other elements of the package would move with the price, including industry assistance and emissions trajectories "which also affect the value of the revenue".

"You can't just do a straight calculation of what the impact would be from a change in price," Ms McCulloch told the Senate estimates hearing in Canberra.

But Dr Parkinson did agree with Senator Xenophon that if the world price was lower than $29 there was a "potential fiscal downside" for the commonwealth budget.

"But there's always been that," he told the hearing.

"In the same way that if permanent prices were higher then there was always a sense that you might find there'd be more revenue.

"Then you obviously need to think about returning that to households or think about what you're going to do in terms of compensation."

While Dr Parkinson envisaged the possibility of boosting household compensation he insisted the proposed assistance wouldn't be cut - even if the carbon price was lower.

That would mean households would benefit from a windfall gain because consumer prices would also rise less than expected.

"If the cost of permits is lower the extent to which households are compensated or over-compensated becomes larger," Dr Parkinson said.

"So it's akin to a (further) tax cut."

Treasury estimates households will experience a cost increase of $9.90 a week as a result of the carbon tax. But they'll receive, on average, $10.10 a week in assistance.

The compensation will be delivered in the form of tax cuts and increased welfare payments.

Senator Xenophon later asked for confirmation that the assistance wouldn't be cut even if the international price "collapsed" and businesses passed on lower costs than forecast.

"That (reducing assistance) has been quite clearly ruled out by the prime minister," Finance Minister Penny Wong told the hearing in response.