The public service travel bill has blown out by $75 million since the Coalition returned to government.

At the same time as 15,000 jobs were cut from government departments, the public service travel bill grew from $502.9 million in 2013-14 to over $575.7 million in 2015-16.

That figure includes $423 million for air travel, $19.9 million worth of car rentals and $132 million for hotels and other accommodation.

The figures represent annual growth of about 15 per cent.

Defence is the biggest spender, racking up more than $171 million in bills in 2015-16, nearly $20 million more than in the previous period.

$25.4 million of that amount was spent on business class airfares.

The Immigration and Border Protection portfolio spent $33.8 million, including $23.2 million on flights, much of which went to the operation of Australia's offshore immigration detention network.

The Human Services portfolio travel total was $28.7 million, while Foreign Affairs and Trade portfolio spent $28.3 million.

At the other end of the scale, Communications spent just $2.1 million.

The travel bill is predicted to keep growing as the Coalition continues its government-wide decentralisation plans.