Senior Labor figures have closed ranks to isolate a "freelancing" senator who derided a major Royal Australian Air Force project as Malcolm Turnbull's $250 million "new private jet".

Key points: A senior opposition source says Senator Kitching was "freelancing" with her comments

A senior opposition source says Senator Kitching was "freelancing" with her comments The new KC-30 tanker was once flown as a Qantas and Jetstar plane

The new KC-30 tanker was once flown as a Qantas and Jetstar plane The plane will spend most of its time as an air-to-air refueller and freighter

During Senate Estimates hearings last week, Opposition senator Kimberley Kitching questioned defence officials over progress on the purchase of a new long-range tanker, capable of carrying passengers and VIPs.

Australia will soon receive the KC-30A tanker with a VIP interior, which once flew as a Qantas and Jetstar aircraft.

It was bought from the lessor in 2015 and modified by Airbus to include the new interior and tanker parts.

The aircraft will be one of seven RAAF KC-30As and will spend the majority of its time as an air-to-air refueller and freighter, and will only be used for VIP missions when the Prime Minister goes overseas with a delegation.

Sorry, this video has expired Labor senator Kimberley Kitching puzzled senior defence figures with her comments.

Over the weekend, Senator Kitching took to Twitter to claim the Prime Minister was spending $250 million on the soon-to-be-delivered aircraft as a "private jet".

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But Opposition Leader Bill Shorten said he did not agree with his Labor colleague, saying it was reasonable for Mr Turnbull to use the RAAF planes.

"I don't share that view at all," he said.

"I think it's legitimate for the Air Force to have ordered these planes and I think it's legitimate if PM Turnbull has one of them used periodically, not all the time, for his travel."

The ABC has been told the Victorian senator, who entered the Senate in 2016 at Mr Shorten's request, was "freelancing" with her comments.

"She was definitely speaking on her own initiative," one senior opposition source told the ABC.

Privately, senior defence figures are also puzzled by the senator's decision to question the project.

"Labor can't afford to play these games on this topic," a high-ranking military official said on the condition of anonymity.

In a statement, the Opposition has reaffirmed its support for the project, which is part of an upgrade to Australia's Special Purpose Aircraft fleet which is flown by the RAAF.

"The Special Purpose Aircraft fleet plays an important role," a Labor spokesperson told the ABC.

"Getting the right aircraft with the right capabilities matters so we get value for money."

Defence Industry Minister Christopher Pyne has taken aim at the Opposition Leader over Senator Kitching's comments.

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"Shorten caught by an exploding cigar, having being briefed and agreed on the need for a new RAAF plane, now attacking it," Mr Pyne wrote on Twitter.

"I presume he and Senator Kitching will rule out using it if he ever manages to become PM?"

Neil James from the Australia Defence Association says it's unfair to characterise the aircraft as a "private jet".

"Primarily these aircraft are airborne tankers for refuelling, secondly they are transport aircraft, third — they have a great 24/7 communications fit, and fourth because they're long range they can be used for VIP transport when the smaller jets can't," Mr James said.