Julia Gillard could block moves by the opposition to force its own legislation through parliament under the constitution. But doing so would be a risky move, constitutional law experts have told Crikey.

Cross-bench MPs are flirting with supporting the Coalition on some policy issues, while Bob Brown has suggested the Greens could back the Coalition’s more generous paid parental leave and mental health initiatives.

While there is no impediment to passing legislation from the opposition benches, constitutional law expert George Williams says the government could take the unprecedented step of blocking appropriation bills if they do not agree with that legislation. All Labor-aligned cross-bench MPs have told the government that they will not block supply.

“I think it’s unlikely that the government would take that step,” the University of NSW professor told Crikey. “But in a big stakes game like paid parental leave, where the costings between the two policies are so different, then that might be an area where they are willing to have a fight over it.”

Williams says the government could act by requesting that the Governor-General not assent to any legislation passed by the opposition. Convention usually dictates the Queen’s representative approves of any law that moves through both houses.

“It’s a big step to take when parliament has spoken and the government would need to act with real caution,” said Williams. “But if the government wanted to play hard ball they could not advise that assent, leaving the bill in limbo.”

Peter Black, a senior lecturer in constitutional law at the Queensland University of Technology, told Crikey the government will be wary of taking any such steps, as it could land them offside with the independents they rely on.

“You would think the government would be quite reluctant to advise the Governor-General that way and that she would be unwilling to act on such advice,” Black told Crikey. “It would be quite an extraordinary step for the government to take and could precipitate a collapse in the government and an early election.”

Williams says parliament has the power to pass its own appropriation bills — but the government has control of supply. Any effort by the opposition to pass appropriation bills could lead to an incoherent fiscal policy and ineffectual law making, he says.

“It’s normally understood that the government has significant control over the budget because of its importance to the national econopmy,” Williams told Crikey. “I think the bottom line is that parliament can pass whatever laws it wishes, but some of those laws may be unwise or ineffective unless they have the full cooperation of the government.”