The government will be forced to explain the conduct of two of its senior ministers – Angus Taylor and Josh Frydenberg – in relation to critically endangered grasslands at the centre of an investigation involving companies part-owned by Taylor.

A Greens motion to compel the explanation passed the Senate with support from Labor, Centre Alliance, One Nation and Jacqui Lambie on Thursday.

During the next sitting period, the government’s leader in the Senate, Mathias Cormann, will have to explain the actions of both ministers “and how it is not a breach of the ministerial standards”.

He will also have to say whether there has been or will be an investigation into the conduct of Taylor and Frydenberg.

Last month a Guardian Australia investigation revealed Taylor held talks with Frydenberg’s office and senior environment department officials about the protection of the natural temperate grassland of the south-eastern highlands.

At the time of the meetings, Taylor was the minister for cities and Frydenberg was the environment minister. The meetings were requested 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 grassland on a property in the Monaro region of New South Wales 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.

A diverse native grassland at Bibbenluke Common, Monaro region, NSW. Photograph: Rainer Rehwinkel

Emails obtained by Guardian Australia showed that Frydenberg’s office raised the grasslands protection with the Department of the Environment and Energy on 8 March, a day after officials from the same department met Jam Land to discuss potential contraventions of the Environment Protection and Biodiversity Conservation Act.

An investigator from the unit that examines breaches under national environment law was also present at the meeting with Taylor and Frydenberg’s office on 20 March 2017.

After lobbying by Taylor, Frydenberg’s office sought advice on whether protection of the grasslands could be watered down without first seeking the advice of the independent threatened species scientific committee, and whether this could be done secretly.

The Greens had initially hoped to have both ministers examined through Rex Patrick’s proposed inquiry into ministerial conduct, a vote on which has now been deferred until the next sitting period.

But they have been unable to gain support from Labor for a number of amendments they proposed for that inquiry and instead moved a separate motion on Thursday.

“There are serious questions ministers Taylor and Frydenberg need to answer about whether their behaviour complies with ministerial standards,” the Greens’ deputy leader, Larissa Waters, said.

“The Greens motion means we will now get an explanation from the government’s leader in the Senate about his cabinet colleagues’ behaviour instead of the ‘nothing to see here’ we’ve been getting from the PM on this issue.”

The Greens motion noted that ministers must “make arrangements to avoid conflicts arising from their private interests, also having regard to interests held by family members” and they must “not use public office for private purposes”.

Taylor has repeatedly stated he asked for a briefing in his capacity as the member for Hume and the compliance action was not discussed.

He said the purpose was to understand the technical aspects of the grasslands protection and its impact on his constituents.

The natural temperate grassland of the south-east highlands is mainly found on the Monaro plains, which are not in Hume.

However, some communities of the grassland are found in Taylor’s electorate.

Frydenberg has not answered questions put to him by Guardian Australia in recent weeks. He has provided a single statement saying Taylor sought a meeting to discuss the upgrading of the grasslands’ status to critically endangered.

“Following the briefing and since, no changes to the listing have been made,” he said.

Taylor’s sister-in-law, the NSW MP and minister Bronwyn Taylor, has also faced questions in the NSW parliament over whether she sought to influence the state government’s submissions on the upgrading of the grasslands protection.

Documents tabled in the NSW parliament last week revealed Bronwyn Taylor lobbied in a personal capacity the NSW Office of Environment and Heritage to drop a separate investigation into alleged illegal clearing of native grasslands on another property she owns with her husband, Duncan Taylor.

The office ultimately found no evidence that the clearing was illegal.



