The Government's scheme was expected to make $208 million by 2020, but yesterday's midyear budget outlook unveiled by the Treasurer, Wayne Swan, revealed a remarkable turnaround because of lower-than-expected carbon prices and a surging Australian dollar.

In May the Prime Minister, Kevin Rudd, said the trading scheme would be budget neutral and announced more compensation to industry under the scheme.

The Opposition said the billions in compensation it proposed for heavy industry would be paid for by amendments to the scheme with unallocated revenue collected from the sale of carbon permits.

Based on a model by Frontier Economics, the Opposition has said there was $50 billion in revenue not committed to industry and household compensation it could draw on.

But notes accompanying yesterday's release said: "This [the $2.5 billion black hole] underscores the need for caution in designing assistance measures, particularly those that lead to a permanent increase in scheme costs."

The Opposition's resources spokesman, Ian Macfarlane, said yesterday that if the Government's figures were shown to be correct the Opposition might have to reconsider its position on not drawing on the budget to pay for extra compensation.

The first step, he said, was to obtain more detail on the forecasts.

The Coalition has charged Frontier Economics to do more modelling on expected revenue from the Government's trading scheme until 2030 and the costs of the proposed amendments.

These are expected to be published this week.