Jam Land, the company part-owned by the energy minister Angus Taylor and his brother Richard, illegally poisoned critically endangered grasslands in the New South Wales Monaro region, the federal environment department has concluded.

The department has ordered the company to restore 103 hectares of native grassland but Jam Land has avoided a fine and criminal finding.

The decision comes three-and-a-half years after Jam Land poisoned the grasslands on a property in Corrowong.

Angus Taylor is a shareholder in Jam Land, via his family company Gufee, and his brother Richard is one of Jam Land’s directors.

Late on Thursday, Labor called for the prime minister, Scott Morrison to sack Taylor “immediately” after multiple scandals.

“Labor is calling for Scott Morrison to finally show leadership on the integrity crises engulfing Mr Taylor and sack him immediately,” Labor’s environment spokeswoman Terri Butler said.

The Greens said “it is time the prime minister sacked him” and called for Taylor’s conduct in the Jam Land affair to be the subject of a further investigation.

“The conclusion of this investigation does not end concerns about Angus Taylor’s contact with the department during the investigation into Jam Land,” the Greens democracy spokeswoman Larissa Waters said.

The Jam Land investigation has taken more than three years and has been controversial because Angus Taylor sought meetings with senior environment officials about the grasslands while it was under way. The office of the then environment minister Josh Frydenberg subsequently sought advice about whether laws protecting the grasslands could be changed.

The department’s investigation concludes Jam Land illegally used herbicide on up to 28.5 hectares of the critically endangered natural temperate grassland of the south eastern highlands on a property in Corrowong in NSW on or about 30 October 2016.

Jam Land property near Delegate in Southern NSW part-owned by Angus Taylor and his brother Richard. Photograph: Mike Bowers/The Guardian

It orders Jam Land to remediate 103 hectares of the grasslands on the property over a period of six years.

Richard Taylor, one of Jam Land’s directors, said the company was considering seeking a review of the remediation order by the minister, as well as other options for appeal.

“We think the department has disregarded the outcome of a mediated conference of experts. It also disregarded the photographic evidence that we provided to them, which showed that the grasslands did not meet the condition thresholds [for protection],” he said.

Richard Taylor said the company already had 100 hectares fenced off and managed for conservation but this was ignored by the department.

He said the remediation order would mean Jam Land was managing a total of 200 hectares for conservation which would make the 800 hectare farm marginal.

A spokesman for Angus Taylor said “the department’s decision is a matter for Jam Land. Minister Taylor has no direct or controlling interest in the company.”

A Guardian Australia investigation revealed Angus Taylor met with senior environment officials and the office of the former environment minister Josh Frydenberg in March 2017 about the listing of the grasslands while the investigation was underway.

Frydenberg’s office sought advice from his department about whether protection for the grasslands could be weakened in secret. Departmental emails also show Frydenberg sought urgent information about the investigation after it was raised in parliament by an unnamed person.

Notes from a senior official show Frydenberg later indicated he was “keen to see [if] he can accommodate Angus Taylor’s requests” and asked his department for a “how to guide” on changing laws designed to protect the grasslands. The documents do not make clear what Taylor’s requests were.

Angus Taylor has repeatedly stated that he sought the meeting with departmental officials on behalf of constituents in Hume who were concerned about the grasslands listing.

Both he and Frydenberg have said the meeting was focused on the “technical aspects” of the grasslands listing.

Neither minister has said what Taylor’s “requests” to Frydenberg were but Taylor’s spokesman has said the minister “has never asked Mr Frydenberg to change laws governing the clearing of native grasslands”.

The department’s investigation found the use of the poison had a significant impact on the critically endangered grasslands. It found that Jam Land did not refer the activity to the environment minister for consideration and that, at no point, had approval been granted for use of the herbicide.

The company has three months to submit a mitigation plan to the department. It has 20 days to apply to the minister for a review of the decision and 28 days to seek an appeal through the federal court.

Exactly what will be required to restore the 103 hectares will be the subject of an independent expert report.

In a statement, published Wednesday night, the department said:

“The department’s investigation found that a critically endangered ecological community, the natural temperate grassland of the South Eastern Highlands, was significantly impacted by the application of herbicide.

“It is a breach of a civil penalty provision of part three of the act to take an action that significantly impacts a critically endangered ecological community without an approval under the act.

“In responding to the contravention of the act, the delegate to the minister for the environment made a remediation determination on 28 April 2020, requiring Jam Land Pty Ltd as the landholder to mitigate the damage by managing and enhancing the natural ecological values of 103 hectares of native grasslands on the property.

“A remediation determination is a penalty that addresses the environmental impact caused by a contravention of part three of the act by requiring a person to take action to repair or mitigate that damage.

“This compliance outcome is consistent with the department’s published compliance policy.”