Todd Elphick has denied he was responsible for a blaze which nearly destroyed the home owned by the parents of his estranged girlfriend, Natasha Smith, last July.

He pleaded not guilty to two charges of arson in relation to the July 10 fire and is on trial in the ACT Supreme Court.

The jury has heard the pair were involved in a custody battle over their young daughter after their eight-year relationship ended in 2012.

Australian Federal Police forensic chemist Laura Bowen told the court on Monday that petrol and ethanol were detected on several items of Elphick's clothing seized after his arrest, including two pairs of tracksuit pants, a hoodie jumper and a pair of white sneakers.

Petrol was also detected on a jerrycan found in Elphick's car, two carpet samples taken from inside the Griffin Place house and in a soil sample taken near the back fence.

Ms Bowen said no petrol was found on Elphick's his hands.

Another forensics expert had earlier told the court the defendant's DNA had been found on the house's fuse box.

The court heard a former friend of Ms Smith, who was with her the night before the fire, had phoned Elphick after Ms Smith received calls from a private number, and again after she learned the house had been burned.

Ms Smith told her friend she suspected her estranged partner was responsible, the court heard.

Elphick had denied he was involved, but the second conversation was cut short when police arrived to arrest him, the friend said.

The witness denied a suggestion by defence lawyer Ken Archer that she had offered to provide Elphick an alibi, saying she had never met him.

Ms Smith's mother told the court her daughter said Elphick had threatened ''he was coming to burn the house down with all of us in it'' during a phone call in late June.

The defence is expected to begin its case sometime on Thursday.

The trial continues before Justice Hilary Penfold.