THE federal Coalition will try to torpedo more than $6 billion in revenue after shadow cabinet resolved yesterday to add a $2.5 billion tax impost on light crude oil to the list of budget measures it will oppose.

With Parliament to resume after an eight-week break, the success of much of the Rudd Government's May budget now rests with the five Greens senators as well as Nick Xenophon and Family First's Steve Fielding.

The removal of a 30-year excise exemption on condensate - a crude extracted from North-West Shelf gas - is worth $2.5 billion over four years and was the second single biggest revenue measure in the budget.

Shadow cabinet decided to oppose it as part of a blanket strategy to oppose all tax increases contained in the budget for which the Government had no election mandate.

These include the $3.1 billion tax increase on alcopops, the $555 million luxury car tax increase, and changes to the seniors card income test, worth $85 million.

The Coalition will also oppose the $300 million decision to increase the income thresholds at which the Medicare levy surcharge applies, despite this being a tax break for middle-income earners.

The condensate decision was a victory for the deputy leader, Julie Bishop, who said the tax would increase domestic gas prices in her home state of Western Australia.

The Liberals are using the claim as a weapon in the campaign for the WA state election on September 6. Others in shadow cabinet argued the measure was worth too much money to oppose and Brendan Nelson said last week there was no evidence it would increase gas prices.

Woodside Petroleum has been lobbying hard against the excise and is expected to make mention of its impact, along with that of the emissions trading scheme, as it issues its half-yearly statement on Wednesday.

The Prime Minister, Kevin Rudd, accused the Coalition of being "grossly irresponsible".

"Are they going to puncture a … $6.1 billion immediate black hole into the budget at a time of global economic uncertainty?" he asked.

The Liberal senator Mitch Fifield said the budget could afford the revenue loss.

Meanwhile Coalition members are also expected to vote today to block changes to the "Birdsville" amendment, which lowers the bar for small business to prove a big business has abused its market power.

with Jessica Irvine