A JURY heard Bradley John Murdoch explain for the first time how it was "impossible" for him to have killed Huddersfield backpacker Peter Falconio.

In private conversations played to the Northern Territory Supreme Court, Murdoch told his parents and two friends that he could not have been the man who ambushed Mr Falconio and his girlfriend Joanne Lees in central Australia on July 14, 2001.

Maintaining his innocence and littering his conversation with expletives, Murdoch said his DNA had been "planted" on the evidence that was being used against him.

"They're trying to build a case up with all the lies and the rest of it," Murdoch said.

The court in Darwin sat in silence as the conversations were played.

Murdoch, normally sitting behind glass in the dock, was absent for personal reasons.

He has denied the murder of Mr Falconio, 28, of Hepworth.

He has also pleaded not guilty to the abduction of Miss Lees, formerly of Almondbury.

The seven short, taped conversations, between October 2002 and March 2003, were between Murdoch and his parents Colin and Nancy Murdoch, girlfriend Jan Pittman and friend Peter Jamieson.

In October 2002, he told his parents it was going to be a "hard, long battle", then reassured them: "We're getting there. We'll do all right."

He accused his former business associate James Hepi, who gave dramatic evidence against Mr Murdoch last week, of planting his DNA on evidence.

"They've got a sample of this DNA, but they're trying to construct the case against me because nothing else adds up," he said.

"It was impossible for me to be there."

Murdoch, a 47-year-old mechanic from Broome, has sat in court for the past month hearing the prosecution case against him.

The couple were allegedly ambushed on the Stuart Highway about 300km north of Alice Springs.

In November 2002, Murdoch told Ms Pittman he was 1,700km away, at Fitzroy Crossing in Western Australia, the night after Falconio disappeared.

It would have been "impossible" for him to have driven from the crime scene to Fitzroy Crossing in that time.

"If I was supposed to have done it from 11 o'clock to 12 o'clock, by the time she reckons this bloke has hunted her, 20 hours later I've done 1,700km through some of the roughest bloody dirt in Australia.

"Impossible."

Murdoch also said he had bought a new canopy for his vehicle about a month before Falconio disappeared.

"That knocks it on the head that I went and did something and then I built a new canopy," he said.

The court heard Murdoch was a large, tidy man with no front teeth.

Mechanic Brett Duthie, a friend and former boss of Murdoch, said he worked part-time at his Broome business, West Kimberley Diesel, between 1998 and January 2001, and at one time lived in a caravan outside the workshop.

He described the former mechanic and truck driver as "quite a big man, large heavy man", with short hair, who was tidy.

"Brad has no front teeth, that was quite distinctive if Brad opened his mouth or smiled," he said.

"Did he smile a lot?" prosecutor Rex Wild, QC, asked.

"No, not a lot. I suppose if he was having a joke or something he would ... smile."

Mr Duthie said when Murdoch spoke he would "try and hold his lip to cover the missing teeth".

"It didn't actually ... come up why he had no front teeth.

"Brad said in one conversation it was just from driving trucks, taking amphetamines at the time which was many years ago."

Murdoch had tattoos on both arms, and would normally wear blue singlets and shorts, and thongs, he said.

He was a "capable" cook, and his vehicle was set up so he could make meals on the side of the road when he drove long distances, he said.

The trial continues.