The democratic process was at least partly responsible for the Defence department's failure to achieve key goals and targets in the current financial year, departmental secretary Ian Watt has said.

Defence has been savaged for underspending its financial year budget by $1.6 billion and for not signing off on as many major project acquisition programs as expected in the past 11 months.

Dr Watt told yesterday's Senate estimates hearing the election campaign and subsequent hung parliament had led to a longer-than-expected period of caretaker government.

For an extended period it was impossible to obtain government approval for major capital investments and other programs.

This argument was challenged by Opposition senators on the Foreign Affairs, Defence and Trade Committee Opposition defence spokesman, Senator David Johnston, and committee chair, Senator Russell Trood.

''Which projects would have been ready to go if the election hadn't been called?'' Senator Trood said.

Senator Johnston cited criticisms of Defence's performance outlined in a budget analysis by Australian Strategic Policy Institute Dr Mark Thomson last week.

Dr Thomson had said unless the current Defence Capability Plan Force 2030 was subject to a comprehensive audit a train wreck would result.

The plan was only updated last December.

Dr Watt took umbrage at Dr Thomson's language and rejected the analyst's claims the level of savings outlined in the Defence budget was ''not credible''.

''We have not been able to fully meet the expectations of the white paper framers,'' he said.

''That does not, however, mean it is either unachievable or should be torn up.''

He said in 2009-2010 Defence had approved 28 major capability projects.

''We are tracking towards 26 major capability projects to be approved in 2010-2011,'' he said.

While admitting this was fewer than had been planned, Dr Watt said a number of factors had come into play.

In addition to the vagaries of the democratic process, bad weather, the failure of defence industry to deliver products on time and high degrees of technical difficulty had all played their part.

Another factor, and one which may come into play as early as this week with the arrival of the first of five air-to-air refuelling tankers being purchased at a total cost of $1.9 billion in Queensland today or tomorrow is the need to mitigate technical risks.