More than a third of $425.5 million in Defence spending announced yesterday is to go on a rugged all-weather ''multi-purpose offshore support vessel'' the navy will hand over to Customs Service fleet - at no charge - in less than five years.

The balance of the Defence spend is for a sixth C-17a Globemaster priced at $280 million and $15.5 million towards the next production run of Bushmaster protected mobility vehicles.

''It must be nearly budget time,'' Australian Strategic Policy Institute analyst, Andrew Davies, said yesterday. ''Defence is spending money again.''

The navy's new ship, the $130 million Skandi Bergen, has been bought at this time to provide extra ''amphibious lift'' capability after the embarrassing failure of the amphibious fleet early last year.

''Amphibious lift ships'' are those the ADF uses to move personnel and equipment. They are crucial in times of natural disaster and, when Cyclone Yasi was bearing down on Queensland early last year, the navy found it had none fit to put to sea.

Designed for operation in arctic waters and the North Sea, the Skandi Bergen is not a typical amphibious support ship. She is a sister ship to the Australian Customs Vessel Ocean Protector, also a former Skandi Bergen, that entered service with the Commonwealth last November.

The new ship is due to undergo sea trials in May and will enter Australian service - with a civilian, not a navy crew - around the middle of the year.

While the timing of the buy has been determined by the short-term needs of the navy, the actual capability is all about the long-term needs of Customs and Border Protection.

Mr Davies said it is an approach that makes a lot of sense. ''I have felt for a long time government should be taking a whole-of-nation approach to these questions,'' he said. ''Defence has way more horsepower in acquisitions [than Customs and Border Protection]. This is a good thing.''

He does not believe using Defence money to buy a ship that will go to Customs when the navy's two landing helicopter docks come on line is robbing Peter to pay Paul. ''It doesn't really matter. One arm of government paying another arm of government is not an issue - it is a question of buying national capability.''

Defence takes the same view. ''The vessel is owned by the Commonwealth and will be transferred from Defence to Customs for use in 2016,'' a spokeswoman said.

It appears likely Customs will return the Ocean Protector, whose charter is due to expire at that time, to its owner, DOF Subsea, in 2016 when the navy hands over the Skandi Bergen. When ACV Ocean Protector was kitted out for her Customs role by Forgacs in Newcastle last year, she was given a new deckhouse module for additional personnel and holding suspected illegal immigrants and suspected illegal fishermen. The module, which also incorporates a medical centre, could be transferred from one vessel to the other given they are close to identical.