Senator Milne, who first flagged supporting the change and then announced her party's opposition after losing the debate in her party room, could now face pressure for a second U-turn, this time led by a grassroots members' revolt. Greens leader Christine Milne. Credit:Alex Ellinghausen NSW senator Lee Rhiannon is pushing to overturn the stance amid what one Greens member called ''despair'' across the green base. A meeting has been called for Saturday at the Sydney Mechanics Institute, where NSW branch members are expected to advocate a return to the party's original position in the interests of policy integrity. In a sign of the intense divisions over the issue, Senator Rhiannon has invited members to have their say, even though the policy has been finalised, setting up a situation in which the party room has one policy and the membership another.

The Greens' constitution in NSW means Senator Rhiannon could be compelled by the membership to vote contrary to her leader, although that would have to come from a formal council meeting. Despite that risk, Senator Rhiannon used a party-wide email to declare she was ''interested to hear from members'' about the issue. Last week Senator Milne said the party would block the increase because the government would not use the money raised to invest in public transport, or to cut fuel subsidies for large mining companies. But Greens inside the party room and in the broader movement conceded that the main reason for opposing the increase was ''political''. The main advocates of the change were Deputy Leader Adam Bandt, Ms Milne's fellow Tasmanian senator Peter Whish-Wilson, and West Australian senator Scott Ludlam.

A senior Greens source called it politics over policy. ''They just can't come at giving Tony Abbott a win, even where it is consistent with our own policy,'' the exasperated member complained. The decision appears to have doomed the $4 billion budget measure because the Coalition had been relying on support from the Greens to get it through the Senate. It was not an unreasonable expectation. Historically, the Greens have favoured higher relative prices for polluting fossil fuels. Greens senators have received ''stacks'' of emails from disappointed constituents over the reversal, as well as official correspondence from at least one state branch protesting against the decision.

Loading The fight over petrol comes as some in the renewable energy sector expressed concerns over the Greens' handling of the Clive Palmer compromise to ditch the carbon tax but keep the renewable energy target, the Clean Energy Finance Corporation, and the Climate Change Authority. Follow us on Twitter