Australian defence force exercise Chong Ju at Puckapunyal. Photo: Joe Armao

The Defence Department has destroyed 85 surplus missiles worth $23 million in the past 18 months.

But the department says getting rid of an average of one missile a week at an average cost of $271,000 per missile is an integral part of national security. 

The explosives were a mix of air-to-air and air-to-surface missiles used by air force and navy fighter aircraft and helicopters as well as air defence and anti-tank missiles used by the army. 

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"Holding sufficient missiles in stock to deal with potential operational scenarios serves as a deterrent to those operational scenarios being realised," a Defence spokesperson said. 

"The weapons are not considered wasted.

"The number of missiles in inventory is determined by the potential operational scenarios they may be required for and is typically higher than the baseline training requirement." 

The missiles were destroyed by being blown up. But they were not blown up by being fired, as missiles were made to be. 

Despite this, the department said the destruction of the missiles provided training to Defence's explosive ordnance disposal teams.

"Missiles can become surplus if the platform they are procured to complement is no longer available, the mission set is no longer required or the missiles themselves have reached their shelf life," the spokesperson said.

"Some of those referred to were surplus to requirements, some of them were shelf-life expired."

While the spokesperson said the shelf life of Defence's missile stock was "factored into training planning", the department did not answer a question about what its procedures were to marry surplus weapons with training.

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Academic Derek Woolner, from the Australian National University's strategic and defence studies centre, said Defence had been criticised in the past because personnel were restricted in what munitions they could access for live fire exercises.

Mr Woolner said computer simulators, while effective, were "not quite like using the real thing".

Another 36 missiles worth $9.175 million were disposed of by being sold.

The department revealed its missile disposals in an answer to a question on notice asked by Senator Penny Wong.

Last month Defence rejected outright a rumour the department had disposed of millions of dollars in pistol ammunition because it had been malfunctioning.