A senior cabinet minister has dismissed as “ridiculous” reports criticising Tony Abbott for the use of an RAAF plane to fly to Melbourne, where he attended a birthday party for a major Liberal donor.

The prime minister on Sunday night joined the birthday celebration of mining millionaire Paul Marks, brother of former federal Labor candidate Samuel Miszkowski, who together were linked to donations of almost $1m to the Liberal party in 2013-14.

Miszkowski, who sold his house to run for Labor in the 2007 federal election but became disenchanted with the party after Kevin Rudd forgot his name at a press conference, was also at the party, Guardian Australia has confirmed.

The Herald Sun reported Abbott joined about 45 people at the party at the exclusive Huntingdale golf club.

Small business minister Bruce Billson said Abbott had meetings in Melbourne and Sydney on the same day.

“Let’s not get stuck into the guy for having a moment with some acquaintances,” he told Sky News, adding the story was a waste of a front page.

Independent senator Nick Xenophon says the prime minister must explain why he didn’t use a cheaper commercial flight.

“These are legitimate questions for taxpayers,” he told reporters in Canberra.

Labor’s Pat Conroy said the flight appeared to be an appalling waste of taxpayer money but that Abbott could clear up the issue by releasing details about the genuine work aspects of his trip.

“While he was sipping on chardonnay and enjoying crayfish, how is this serving the Australian public?” he told reporters in Canberra.

Miszkowski, who told Fairfax he was “left hanging like a shag on a rock by the ALP when I ran”, and his brother Marks, who has changed his name, were both linked to some of the biggest single contributions to the Liberals in 2013-14.

Marks personally gave $250,000 and is the executive chairman of mining company Nimrod Resources, which gave $500,000, according to the Australian electoral commission.

Miszkowski is the financial controller of Mist Consulting, which gave $200,000.