She accused the gallery of a “grossly unfair depiction of a prominent figure in Australian society” that only serves to “perpetuate stereotypes and undermine the achievements” of Rinehart.

Mitzevich was so concerned about the mounting complaints that he warned Burke’s office they could be raised in parliament by Dutton.

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“I have responded to the complaints, however, felt I should elevate this to you in case these matters are raised by the leader of the opposition (who is close to Mrs Rinehart), so the minister can be prepared,” Mitzevich wrote.

Dutton did not raise Rinehart’s response in parliament, but he has had a long association with the mining magnate, whom he has called a “dear friend” and “great Australian”.

In November, Dutton took a private jet to sit next to Rinehart as her star guest at her party at Roy Hill mine in the Pilbara region of Western Australia. In March, he flew to Perth for Rinehart’s 70th birthday party. Then in June, Rinehart hosted two private political fundraisers for the opposition leader, The Australian Financial Review reported last week.

By May 2, more than 26 emails had been exchanged about the furore, according to the FOI documents.

At least three complainants wrote in near exact terms that they and thousands of others would not be where they were today if not for Rinehart’s generous contributions out of the “kindness of her heart”.

Another complaint said the portraits “should be taken down and redone immediately”.

“Mrs Rinehart is one of the humblest, caring, down to earth people I have had the pleasure of spending time with,” wrote another.

One demanded the “insult” be met with a formal written apology from the NGA expressing regret for the “disrespectful portrayal”. Another described the works as “tawdry commercialism at best, cheap shots, resonating from a platform of ignorance”.

“In my opinion, this artwork should be removed and replaced with a portrait celebrating her and all she has done and continues to do for Australia,” one supporter said.

The contents of Rinehart’s original complaint have not been released, and all but one name of the complainants has been withheld, but they include elite Australian swimmers competing at the Olympics. Rinehart is expected to appear poolside in Paris this weekend.

Rinehart has paid more than $40 million in sponsorship to Australian swimmers through Swimming Queensland and her company’s Hancock Prospecting Swimmer Support Scheme.

Olympic gold medallist Kyle Chalmers and Kevin Hasemann, one of the top officials in Australian swimming, have previously confirmed their part in leading the charge against the Namatjira portraits.

“Mrs Rinehart has proven herself to be a great Australian, and we respectfully urge you to reconsider the inclusion of these portraits in your galleries,” Hasemann wrote on May 2.

Namatjira’s exhibition, which closed last Saturday, also included satirical portraits of Queen Elizabeth, AFL star Adam Goodes and former prime minister Julia Gillard. The Indigenous artist and Archibald Prize winner is known for his rawness and sardonic treatment of subjects.

Mitzevich quoted Namatjira in his response to complainants informing them that the gallery would not be removing the paintings.

“Whether you’re a mining magnate, sportsperson, politician, musician, whatever – we are all equal here,” Namatjira wrote of the exhibition.

In the NGA’s defence, Mitzevich said the National Gallery’s remit was to be a reference point for art and art history.

“Since 1973, when the National Gallery acquired Jackson Pollock’s Blue poles, there has been a dynamic discussion on the artistic merits of works in the national collection, and/or on display at the gallery,” he said.

“We present works of art to the Australian public to inspire people to explore, experience and learn about art.”

Rinehart and Dutton were contacted for comment.

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