Defence will spend a further $19 million on a highly secret and ultra-powerful ''jamming'' technology for 12 of its Super Hornets but has stopped short of publicly committing to buying the capability.
The RAAF has already paid a $35 million premium to have the planes hardwired as ''Growlers'' while they were still on the production line.
It is believed the full cost of the Growler conversion would run to about $500 million, including the $54 million that has already been spent.
If Defence does not sign off on the full upgrade - which involves sending the 12 planes back to the Boeing production line in the United States - the $54 million will have been wasted.
Sources said yesterday the government was already ''in for a penny, in for a pound'' but would not publicly commit to a $500 million spend in the current climate of restraint and budget cutbacks.
''If Defence said they were going to spend the whole $500 million the opposition would almost certainly ask where the money was coming from,'' one insider said.
There is almost universal agreement the Growler capability - which the US has not shared with any other allied air force - is cheap at the price.
''This [the decision to spend $19 million on long lead-time electronic equipment for the electronic warfare system] is a good start,'' former chief of air force and now chairman of the Williams Foundation think tank, Air Marshal Errol McCormack, said. ''It needs to be followed up on.''
He said if the equipment was not ordered now there was a real risk that Boeing might close the production line and the opportunity would be lost.
Defence Minister Stephen Smith said the spend was necessary ''to ensure Australia continues to have access to the Growler technology''.
He said a second Shadow 200 unmanned aerial vehicle, valued at about $40 million, should be delivered before the end of June for deployment to Afghanistan.
Growler technology gives the Super Hornets the ability to jam the electronics systems of other aircraft, land-based radars and communications systems. The US used Growlers to seize control of the skies over Libya last year.
Australian Strategic Policy Institute analyst, Andrew Davies, said Growlers had an impressive capability which complements that of the soon to be acquired Joint Strike Fighters.
''This is a sensible, and not unexpected decision,'' he said. ''While the JSF has a formidable electronic warfare capability, the Growler can cover a broader range of frequencies.
''There would certainly be plausible scenarios where the two systems working together would be even more effective than either one on their own.''