The Nationals fear vital foreign investment will be scared off, and jobs lost, if the government does not finalise approvals for the Adani coal mine before the election is called.

Prime Minister Scott Morrison is widely expected to call an election this weekend for May 18, which would call a halt to any government decision on the controversial mine.

Environment Minister Melissa Price received a formal briefing from her department on remaining issues with the Carmichael mine in central Queensland last week.

The briefing was informed by scientific work on groundwater management by two agencies, the CSIRO and Geoscience Australia, with input from Adani.

Mr Morrison described the remaining issues as “quite minor in the scheme of the broader approvals provided already”, but refused to be drawn on when a decision might be made.

In a Senate estimates hearing on Monday, Labor senators quizzed officials on a letter – obtained by The Courier-Mail – from Liberal National Party senator James McGrath to Ms Price in which he said he would publicly call for her resignation if she failed to treat the Adani project fairly.

Environment department secretary Finn Pratt said the first he was aware of the letter was when opening his morning media clippings.

Government minister Simon Birmingham said in any case Ms Price would be making her decision “consistent with her legal obligations” and in accord with the expert advice provided to her

“The looming election should have no bearing in relation to the decision,” Senator Birmingham said.

Nationals senator John Williams, however said the timing of the election should weigh on the minister’s mind.

“What worries me is if it’s not ticked off, Labor will never tick it off if they win government – goodbye Adani,” he told Sky News.

“What message does that say to foreign investors in Australia – come over and spend hundreds of millions of dollars and then get knocked backed because of some regulation.”

Nationals minister Darren Chester was also keen to see the project approved by the end of the week, in the interests of regional jobs and revenue.

“We’ll go into an election campaign, I’m assuming, with a decision made on the project,” he told ABC television.

Finance Minister Mathias Cormann said Senator McGrath’s public intervention wasn’t helpful.

“I would have much rather if he had not made that sort of communication but he has and we are where we are,” he told a Senate hearing.

Anti-Adani protesters disrupted a business lunch speech by Mr Morrison in Brisbane on Monday.

Labor leader Bill Shorten said the environment minister was being bullied.

“This is a failure of ethics in government at the highest level.”

Mr Shorten said in government he would be guided by the “best science and the law of the land” on the Adani approvals, but no taxpayer money would go to the project.