Labor has called on the prime minister to investigate what it calls “shocking allegations of misconduct” involving Angus Taylor and Josh Frydenberg’s office.

A Guardian Australia investigation has revealed Taylor met with environment officials about an issue at the centre of a compliance action involving a company he part-owns.

Following those talks, the office of the then environment and energy minister, Frydenberg, canvassed whether protections for a critically endangered grassland at the centre of the compliance action could be watered down and if it could be kept secret.

Labor’s environment spokeswoman, Terri Butler, said Scott Morrison “must investigate the serious matter of a potential breach of the ministerial code of conduct”.

The Greens’ leader, Richard Di Natale, said the episode showed the “urgent need” for a national integrity commission and said the party would move a Senate motion demanding the government explain the actions of both ministers when parliament returns next month.

Guardian Australia has sought comment from Morrison.

“Today’s shocking allegations of misconduct against senior Liberal government ministers are an indictment on the Morrison government,” Butler said. “It appears that Angus Taylor breached the ministerial code and met with the environmental department while it was investigating the company to which he has links.

“It also appears that Josh Frydenberg’s office sought advice as to whether he had the ministerial powers to weaken grasslands protections in secret, then sought to conduct a review into agriculture and the EPBC Act.”

Butler said Taylor and Frydenberg needed to explain exactly what had occurred.

“It is not good enough that Mr Frydenberg was considering deploying ministerial powers to weaken environmental protections in secret, after arranging meetings for his colleague who has a conflict of interest in the matter,” she said. “This is not the first time questions have been asked in relation to Angus Taylor’s conduct and potential conflicts of interest.

“Australians will not let them get away with it, and neither should Scott Morrison.

“It’s imperative that Australians can trust their representatives not to interfere in matters that relate to their personal financial interests.”

Di Natale said: “It is appalling to consider that Angus Taylor would use his privileged position to approach departmental staff and government ministers about an issue that directly affects his business interests.

“We will be ensuring that Angus Taylor and Josh Frydenberg explain their actions to the parliament when it resumes next month.

“This whole episode demonstrates again how urgently we need a federal Icac that can investigate these sorts of behaviours that undermine faith in democracy.”

Guardian Australia’s investigation, published on Wednesday and Thursday, revealed that Taylor, then the minister for cities and digital transformation, sought meetings with departmental officials to discuss the 2016 listing that protects the natural temperate grassland of the south-eastern highlands as a critically endangered community under federal environment law.

The meetings were requested in 2017 via Frydenberg’s office. At the same time federal and state investigations were under way into the alleged poisoning of 30 hectares that contained the grass species on a property in New South Wales’s Monaro region owned by Jam Land Pty Ltd.

One of the directors of that company is Richard Taylor, the minister’s brother, and the minister himself holds an interest in the firm via his family investment company, Gufee.

Taylor has denied making any representations regarding the compliance action and said he received a briefing in his capacity as the member for Hume for the purpose of understanding the technical aspects of the revised listing and its impact on those affected, which included many constituents.

The critically endangered grasslands are mainly found on the Monaro plains, which are not in Hume. However, some communities of the grassland are found in Taylor’s electorate.

Following lobbying by Taylor, Frydenberg’s office sought advice on the full extent of his ministerial powers and whether he could override scientific advice and remove the “critically endangered” listing.

Environment groups said the revelations raised serious questions about the integrity of Australia’s national environment laws.

“The Australian Conservation Foundation calls on the parliament to investigate these serious questions relating to the actions of Josh Frydenberg and Angus Taylor,” Basha Stasak, the ACF’s nature program manager, said. “Australians need to be sure breaches of environmental law are investigated free from political interference and that scientific listing decisions are not tampered with.”

Evan Quartermain, the head of programs at the Australian arm of Humane Society International, said investigations by federal environment officials into illegal agricultural land clearing were extremely rare. He said both ministers needed to “explain their conduct”.

“It’s atrocious that we have ministers seeking to remove protections while an investigation mysteriously drags on for years,” he said. “This is a single incident on land owned by very well-connected people who seem to believe they are above the law, and I don’t think it’s a coincidence that an EPBC Act agricultural review was launched shortly after the meetings, or that other threatened habitat listings have been avoided since they occurred.”