AN Australian mechanic pleaded innocent today to murdering British backpacker Peter Falconio on an Outback highway more than three years ago.

Bradley John Murdoch pleaded innocent in the Northern Territory Supreme Court in Darwin to murdering 28-year-old Falconio, of Hepworth.

He also entered an innocent plea to the charge of assaulting and illegally detaining the traveller's girlfriend, Joanne Lees, formerly of Almondbury.

Although Murdoch has been in detention for months, he had never been asked to enter pleas to the charges.

He was formally arraigned today and his trial is due to start in the Northern Territory Supreme Court in early May.

Murdoch faces a mandatory life sentence if convicted of murder.

Murdoch is suspected of shooting Mr Falconio after flagging down the camper van the British backpacker was travelling in with Miss Lees on July 14, 2001.

Prosecutors say Murdoch also punched Miss Lees and bundled her into his pickup truck, but she managed to escape.

Mr Falconio's body has never been found, despite a police search covering an area of the Outback the size of France, involving aircraft, police on motorcycles and Aboriginal trackers.

His disappearance sparked one of the biggest-ever police investigations in Australia.