The future of Clive Palmer’s $6bn China First coalmine now rests on two separate environmental decisions by the federal minister Greg Hunt, as the mining magnate prepares to wield his powerful four-senator balance of power voting bloc from next July.

On Friday, the Department of Environment posted a determination on its website that the mine would have to comply with new laws requiring a cumulative assessment of the impact it – and other coal mega-mines in the Galilee Basin – would have on water resources, before it could proceed.

And China First has also recently completed a federal environmental impact statement, which the federal government must assess and approve.

The two outstanding federal approvals are the final government hurdles for the mine, for which the Queensland co-ordinator general, Barry Broe, gave conditional approval in August.

The mine, which is being developed by Palmer’s company Waratah Coal and aims to export 40m tonnes of coal a year for 30 years, would significantly affect the 8,000-hectare Bimblebox nature refuge, Broe found.

But he said he was “satisfied that Waratah [the company proposing China First] has adequately assessed the environmental values of the [reserve], identified impacts, proposed migration measures and committed to provide compensation for significant residual impacts by way of offsets”.

Having won two, and possibly three, Senate positions in the September election and having signed an undisclosed memorandum of understanding to win the support of a fourth – the Motoring Enthusiast party’s Ricky Muir – the Palmer voting bloc will determine the fate in the new Senate of any government legislation opposed by Labor and the Greens. (PUP’s West Australian Senate candidate was elected on the first count, but the Senate vote in that state is being recounted.)

Paola Cassoni, part-owner of the private Bimblebox nature refuge, has questioned whether the public can have confidence in the federal assessment processes given the powerful position Palmer will hold in parliament.

“This situation is ludicrous. How on earth can we believe that the negative impacts from this mine are being properly considered when the Abbott government has a motive to win favour of Clive Palmer?” Cassoni said.

“If the federal government approves this mine, the Australian public will have good reason to suspect that it is a politically motivated decision. The only sensible thing to do will be to put a halt on this assessment right now, until proper transparency measures can be put in place.”

The government’s looming decisions on Palmer’s business interests come as the independent senator Nick Xenophon says he will try to move a private member’s bill to force the disclosure of the deal done between Palmer’s party and the Motoring Enthusiasts.

“The public has a right to know what was in the MOU,” Xenophon said. The Greens have also demanded disclosure of the MOU.

On 27 September, Hunt announced he had determined that 47 projects – including China First – would require federal environmental assessment of their impact on water resources. Determinations for most of those 47 projects had already been posted on his department’s website.

But on Friday morning the decision on China First had not.

Nui Harris, the managing director of Waratah Coal, told Guardian Australia: “The minister is still considering the issue. We have zero clarity on this and we really need some certainty. We are happy to abide by the legislation but we need to know whether it applies to us and what it requires us to do.”

By Friday afternoon a decision had been made and the referral was posted.

The website for the China First project says construction is scheduled to start “in early 2014” – a timeline that would be difficult to meet if detailed new studies into its water impact are required.

Non-governmental assessments have raised concerns about a large and "unacceptable" impact on groundwater and surface water from mines planned for the Galilee region.

Palmer has said he supports the abolition of the carbon tax – the Abbott government’s first legislative act – but thinks it should be retrospective, so that those liable to pay it – including him – would get a refund or have their liability waived. Abbott has ruled this out.

In July, the Clean Energy Regulator charged Palmer $6.2m for refusing to pay carbon tax owed by his private company, Queensland Nickel, and the magnate has filed a petition in the high court claiming the tax is unconstitutional.

The Bimblebox nature refuge is a former cattle property bought by conservationists in 2000. Palmer says his mine would support 3,500 jobs during construction and 2,325 more when operational.