According to a leaked draft of the 2013 defence white paper, just two Lockheed Martin JSFs will be delivered to Australia by 2020.

This strongly indicates that the government will need to buy rival Boeing F/A-18F Super Hornets, which are cheaper but older and less stealthy than the JSF.

''By the end of this decade, the ADF will take delivery of three Air Warfare Destroyers, two Landing Helicopter Dock amphibious ships and the initial two F-35A Joint Strike Fighter aircraft,'' the white paper states.

While switching to the Super Hornets would not be a blow to the budget - each plane costs about $40 million less than each JSF - it may mean money is wasted because the government would lose economies of scale on training and maintenance by operating two different types of fighters. And experts say the Super Hornet would be challenged by the growing air combat capabilities of some of Australia's neighbours.

The white paper draft states that the government ''remains committed'' to acquiring the JSF but makes no mention of the next batch of 12 planes, expected about 2020. This appears to confirm what the Defence Minister, Stephen Smith, has hinted at and many experts have suspected: that Defence will replace some of the retiring Hornet aircraft with Super Hornets and end up with a mixed fighter fleet rather than the 100 Super Hornets originally proposed.

Mr Smith has already asked the US about the price and availability of more Super Hornets.

The opposition defence spokesman, David Johnston, said the government had broken its pledge in the 2009 white paper to buy 100 JSFs, which would have "provided regional domination out to 2030".

"The revelation in the 2013 defence white paper that this promise has been reduced to just two aircraft (by 2020) is a further testament to Minister Smith's incompetent handling of the defence portfolio," he said.

Analysts say the JSF is the best fighter on the market, although many say the Super Hornet will probably suffice. Andrew Davies, a senior analyst at the Australian Strategic Policy Institute, said the JSF was "far stealthier and has a much more powerful and integrated set of senses than the Super Hornet has".

He said Australia would benefit from "economies of scale on training and maintenance" by having a single type of airforce rather than a mixed fleet.

''Nonetheless, the Super Hornet is still frontline equipment with the US Navy and a powerful air combat capability,'' he said.

Sam Roggeveen, an analyst and editor of the Lowy Institute's Interpreter blog, said the Super Hornet would represent a compromise but added: "I would argue we don't need the JSF yet."

Former defence minister Brendan Nelson, who bought the existing 24 Super Hornets, said a mixed fleet should give Australia what it needed, given other governments were hit by budget constraints.

"If the government did choose to [buy Super Hornets], Australia would still have extraordinary air combat capability and would be well-placed in relation to our strategic competitors," he said.

But Peter Goon, a former RAAF engineer now with the independent think tank Air Power Australia, said Australia was "already outmatched in the region" on air combat. "If you send out Super Hornets against the Sukhoi Su-35s, few if any of them will come back," he said.

Mr Smith said last week the leaked draft was out of date. The final paper will be released by June.