Australian Greens leader Christine Milne has no doubt "old factional bosses" and union leaders were behind Labor's decision to walk away from the minor party "into the arms of the big miners".

In a sharp rebuke to Prime Minister Julia Gillard, Senator Milne on Tuesday said Labor had effectively ended its agreement with the Greens.

By choosing the big miners, Labor had made it clear it no longer had the "courage or the will" to work with the Greens on a shared agenda for the national interest, she said.

But Senator Milne said the Greens will stand by their pledge to deliver confidence and supply to Labor until the next election so as not to advance the interests of Opposition Leader Tony Abbott.

"The Greens will not add to the instability that Labor creates every day for itself," she told the National Press Club on Tuesday.

"It is a party riven with internal descension and we're not going allow all their internal fights to distract us from getting on with delivering good outcomes for people and caring for the environment."

She said AWU national secretary Paul Howes was "fully behind" Labor's decision to break away from the Greens.

"It's the old factional bosses and the old deals being done in back rooms," she said. "Well, over to them."

Delegates at the AWU national conference on Tuesday approved a plan to lobby the federal government for a gas reserve to make energy cheaper, boost jobs and produce cleaner energy.

The conference also called for an expansion of coal seam gas development in NSW, which is facing a backlash from farmers and environmentalists.

The Greens said the move would cost jobs, not create them, at a time when NSW residents were calling for a widespread ban on CSG exploration.