PRIME Minister Tony Abbott used a taxpayer-funded RAAF plane to fly to Melbourne, where he attended the birthday party of mining millionaire and big Liberal donor Paul Marks.

Mr Abbott joined about 45 people for the birthday party at the exclusive Huntingdale Golf Club on Sunday night.

He started the day in Brisbane and flew to Sydney for the NSW Liberal election campaign launch with Premier Mike Baird before flying with a small group aboard the RAAF VIP plane to Melbourne.

Once he arrived at Huntingdale, the PM shed his tie and mingled with guests, enjoying crayfish and aged Wagyu beef with a glass of chardonnay. He also gave a speech in honour of Mr Marks.

His office defended the use of the VIP plane, with a spokesman advising: “The Prime Minister had other work-related engagements in Melbourne on Sunday. All travel was undertaken within the rules.’’

He did not answer questions on what the work engagements were.

Mr Marks is a major financial supporter of the Liberal Party.

Australian Electoral Commission returns shows he personally donated $250,000 to the federal Liberal Party in 2013-14, while his company, Nimrod Resources Limited, donated a further $500,000.

Mr Marks declined to answer questions about the party, saying “yeah, so?’’ when the Herald Sun contacted him.

“When you have a birthday party it is a private affair. I don’t have any comment about anything,’’ he said.

Assistant Defence Minister Stuart Robert, who is based in Queensland, was also at the intimate gathering. He too gave a speech.

Mr Robert’s spokesman said: “Stuart was at a private function and if he attends a ­private function and there are travel costs involved, he pays for himself. No public money was expended on this private trip.’’

Independent senator Nick Xenophon said the use of the VIP plane was inappropriate if the private function was the primary reason for Mr Abbott’s visit to Melbourne.

“The Prime Minister needs to justify the expense to the Australian taxpayer, and if the justification doesn’t pass the pub test then he or the Liberal Party need to refund the cost of the VIP (plane),’’ he said.

The RAAF maintains several jets for Mr Abbott and senior ministers and VIPs, such as the Governor-General, to use.

The most recent Schedule of Special Purpose flights, which details the costs of the use of the jets, shows it costs about $4000 an hour to run.

The aircraft was used to bring Mr Abbott back to Canberra from Melbourne yesterday morning.

Staff at the Huntingdale Golf Club told the Herald Sun that Mr Abbott was “relaxed and friendly” at the function.

Seafood, including crayfish, featured heavily on the menu, and sparkling wine was circulated with the hors-d’oeuvres.

The club’s waiting staff, including one woman who posed for happy snaps with Mr Abbott and Mr Marks, talked excitedly about meeting the PM.

The golf club remained open to members during the function.

Security provided for Mr Abbott was discreet.

Last August, Mr Abbott was criticised after he told Coalition MPs he was running late because he had to schedule a last-minute visit to the Peter MacCallum Cancer Centre in Melbourne to justify taking the VIP plane down for a Liberal Party fundraiser.

ellen.whinnett@news.com.au