Tony Burke says he has always been sceptical about whether the law has been followed in relation to previous environmental approvals for the controversial Adani coal project in Queensland.

But the shadow environment minister argues that he can’t telegraph a firm disposition about what he might do about the approvals in the future without making a prejudgment that could render any subsequent decision unlawful.

Adani has flared again as a political problem for Labor federally because key unions have made public interventions over the past week arguing that the project should be supported by the Queensland Labor government – interventions that have been leapt on by Coalition frontbenchers who see opportunity to bolster the Morrison government’s political position in central Queensland in the coming election.

While the government is blasting Labor from a pro-coal, pro-jobs position, Labor is also under sustained pressure from the environment movement, the Greens and progressive activist groups to toughen its stance against a project which has become a proxy for action on climate change in the national political debate.

Burke told the ABC on Sunday morning there were “deep problems with the way the Liberal government has handled the environmental approvals”.

“From the environmental end, I have always been sceptical of the project,” Burke said. “I’m concerned about from what’s been reported the government doesn’t appear to have followed the law with respect to the use of the water trigger.”

The shadow minister referenced the sustained pressure from the Greens and environmentalists for Labor to adopt a more definitive position on the project, but Burke said he was not, because of his portfolio responsibilities, in a position to be definitive. “It is unlawful for it to be prejudged.”

In explaining his stance, Burke referenced a long-running saga in the 2000s in which a Liberal environment minister had to overturn his own decision to block a $220m windfarm project to save orange-bellied parrots after a federal court appeal.

Burke has been telegraphing problems with the environmental approvals since taking the portfolio. Last year he called for a federal investigation after sensitive wetlands were contaminated near the Great Barrier Reef after Cyclone Debbie, pointing out that environmental approvals for Adani’s Abbot Point terminal placed conditions on the project, including the downstream impacts on the Caley Valley wetlands.

Bill Shorten signalled publicly, and privately to environmental activists at the start of 2018, that federal Labor was looking to toughen its position against the controversial project.

During that period, which coincided with the Batman byelection in Victoria, the Australian Conservation Foundation gave Shorten legal advice arguing that the commonwealth environment minister has discretion to revoke the Environment Protection and Biodiversity Conservation Act approval for Adani “on at least two grounds”.

The first ground would be “new information of the consecutive bleaching of the Great Barrier Reef, indicating increased sensitivity to greenhouse gas emissions” and the second would be “new information of the insufficiency of offsets for the endangered black-throated finch, indicating a threat to the continued survival of the species from the Carmichael project”.

Shorten pulled back from those public signals of equivocation after encountering internal opposition to toughening up the position on Adani, with some colleagues concerned about the implications for sovereign risk, and also the electoral fallout in central Queensland.