Defence Force members are among Australia's hardest workers, spending long hours each week on the job with extended periods away from home, new figures show.

It has also become harder to hold on to recruits with fewer committed to long-term careers in the armed forces.

The latest Defence census, released on Tuesday, shows 87 per cent of permanent force members in 2007 were male.

They put in long working weeks, clocking in for an average of 48.9 hours per week, although that was down slightly compared to the last survey in 2003 when the average was 49.3 hours.

The data shows 28 per cent work more than 50 hours a week and 14 per cent more than 60 hours.

They also spend long periods away from home, with 81 per cent of permanent members absent from their usual residences for an average of 15-16 weeks, reflecting the ADF's high operational tempo which includes ongoing missions in Iraq , Afghanistan , East Timor and the Solomons.

Only 12 per cent had not been away at all.

Defence personnel are leaving the forces sooner, with the median length of service for permanent ADF members 6.8 years, down from 8.2 years in 2003.

The figures show 59 per cent now serve less than 10 years, compared to 56 per cent in 2003, although 15 per cent had completed 20 years or more service, up from 14 per cent in 2003.

Close to half (44 per cent) of permanent members said they were undecided about how many more years they expected to serve, a marked increase on the 33 per cent in 2003.

Only 15 per cent said they had not considered leaving, a substantial decrease on the 26 per cent in 2003.

Defence members are also getting younger, albeit slightly, with the median age in the latest census 29 years, compared with 29.6 in 2003.

Reservists tended to be older. The median age for an active navy reserve member is 45.7 years, compared to 40.9 for the air force and 32.8 for the army.

Most permanent force members (87 per cent) were born in Australia, unchanged from the 2003 census. Of the foreign-born, five per cent were from the UK or Ireland and two per cent came from New Zealand .

Defence Personnel Minister Warren Snowdon said the data played an important role in framing personnel policies and initiatives that made a difference to ADF members and their families.

The information also allowed Defence to better target its programs, ensuring value for money, he said.

Meanwhile, the federal opposition says the Rudd government has dishonoured the defence community by watering down its promise to provide free medical care to Australian Defence Force (ADF) families.

ADF officers and troops receive free medical and dental care but their families do not.

The government pledged $33.1 million during the 2007 election campaign to provide free medical care to the spouses and children of ADF personnel at 12 clinics.

But last year, Labor announced it would instead use existing health care practitioners to provide basic medical care to families on a trial basis, at a cost of $12.2 million over four years.

Liberal defence science and personnel spokesman Bob Baldwin and colleague Stuart Robert slammed the government's change of heart in parliament on Tuesday, saying it had betrayed the defence community.

"This legislation is a sign the government has failed to deliver on its promise to provide free medical treatment for all ADF families," Mr Baldwin told parliament.

Mr Robert said defence families had been sold short.

"This is a dishonouring of the defence community," he said.

Labor MP Mark Butler said the government had not broken an election promise because it had committed to providing free medical and dental care to ADF families in a progressive fashion.

"We are well on the way on delivering on that commitment," he said.