State of the art pilot-less planes headed for Australia as part of $7billion defence spend

The first of at least four new unmanned surveillance aircraft roughly the size of a Boeing 737 has been spotted on its long journey across the Pacific to Australia.

The Northrop Grumman MQ-4C Triton - also known as the AUS 1 - left a Californian Naval Air Station on Thursday en route to Katherine via Wake Island.

The drone is expected to land at RAAF Base Tindal on Saturday.

It will then be redeployed for maritime patrols to assist Australian Border Force.

Flying under the call sign SCORE47, the drone was last seen on flight tracking software FlightAware off the California coast, heading into the Pacific Ocean.

Defence Industry Minister Pat Conroy last year confirmed the purchase of a fourth MQ-4C Triton as part of a $1.5 billion boost to the RAAF's capabilities.

The long-range aircraft will be piloted from Edinburgh Air Base near Adelaide following a spate of recent unauthorised arrivals by boat into Australia.

About 30 suspected asylum seekers from Pakistan and Bangladesh were detained in February after arriving near Beagle Bay, 130km from Broome.

It comes more than 20 years after Australia first signalled an interest in acquiring the Triton, but has not come without criticism from sceptics.

Despite operating upward of 50,000ft, the Triton drone is vulnerable to enemy attack and reportedly cannot monitor small wooden vessels.

Head of Air Force Capability Air Vice-Marshal Wendy Blyth welcomed the drones arrival and said aircrews had already conducted extensive training.

'The MQ-4C Triton will deliver unprecedented persistence and awareness over the maritime domain in support of the Integrated Force,' he said on Wednesday.

'Uncrewed aerial systems offer enormous potential to capitalise on the opportunities provided by modern payloads and increased endurance.

'This will significantly enhance our ability to persistently patrol Australia's north and northwestern approaches; in the southwest Pacific and south to Antarctica.'