Police have revealed how they used data from number plate recognition cameras and a mobile phone to connect alleged high country killer Greg Lynn to the disappearances of two campers.

Mr Lynn, 57, is standing trial over the deaths of retirees Carol Clay and Russell Hill at a remote campsite in Victoria's Wonnangatta Valley in March 2020.

He has pleaded not guilty to two counts of murder.

Among several police officers called to give evidence in the Supreme Court of Victoria on Wednesday was Detective Leading Senior Constable Abbey Justin, who led the investigation once the missing persons unit took carriage of the case in April 2020.

Senior Constable Justin told the court she eventually obtained the missing pair's phone records as well as a type of telecommunications data that can help investigators determine the location of a handset.

"Essentially the data from the phone connects to towers and can tell where the phone may be," the police officer explained, adding the data wasn't necessarily linked to phone calls.

"It could be applications on a phone that are connecting to cell towers."

The court heard Mr Hill's phone was active near Great Alpine Road at about 10am on the morning after his alleged murder, an area Ms Justin agreed was a "substantial distance" from the Wonnangatta Valley.

When police travelled to the area, they discovered two sets of two motion-detecting cameras linked by about a 20-minute drive along the road.

While the devices were intended to enforce entry fees at an alpine resort, the court heard police used images captured by the cameras on the morning in question to determine the directions of travel of about a dozen vehicles across the 20 minutes leading to 10am.

Senior Constable Justin explained that her focus was on one set of devices that could capture vehicles taking a route consistent with Mr Hill's handset.

“Is there only one vehicle, using that analysis, that you’ve explained to us could’ve been travelling north up Dargo High Plains Road and then turning left onto the Great Alpine Road and going through those cameras?” prosecution counsel Kathryn Hamill asked.

"Yes," Senior Constable Justin replied.

Images of that vehicle, described by the officer as a blue-grey Nissan, were shown to the jury earlier in the day.

Senior Constable Justin told the court she ascertained it was registered to Greg Lynn.

Lynn repaints vehicle

Senior Constable Justin told the court she attended Mr Lynn's residence in Caroline Springs in July 2020 and noticed a Nissan Patrol parked in a side street.

She said the number plate matched the one captured on the motion-detection cameras, but the colour of the vehicle was a "light brown".

Prosecutors have previously alleged Mr Lynn used his trailer to move the retirees' bodies after their deaths, set fire to their campsite and took several further steps to avoid detection, including repainting his vehicle and listing the trailer on Gumtree.

The defence team, on the other hand, contends this was Mr Lynn making "a series of terrible choices" to cover up a pair of deaths that were tragic accidents.

In cross-examination, defence barrister Dermot Dann KC took Senior Constable Justin to a section of a police interview where Mr Lynn said he had painted his car.

"When he was interviewed by police, Mr Lynn told police that since that trip to Wonnangatta, he's painted the car using a 'sandbank' colour," Mr Dann KC said.

"Does that accord with your observation of the car when you attended his home in July?"

"Yes," Senior Constable Justin agreed.

Clay told friends about relationship, court hears

Earlier in the day, the jury was read written police statements from two employees of the Country Women's Association who heard from Ms Clay, a former state president, the day she set out on the fatal camping trip.

Louise Heib's statement described Ms Clay as a close friend who confided in her about her rekindled relationship with Mr Hill.

"Carol told me they were in a relationship together when they were a lot younger, but when Carol started to get serious about the relationship, when they were younger, they broke up and it devastated her," the statement read.

Amanda Darmody was working at the association's Toorak office when she took a call from Ms Clay on March 19, according to a police statement given about six months later.

"I'm pretty sure on the phone she told me she was going away for a couple of days and her phone may break up because she was going out of range," the statement said.

Prosecutors allege Mr Lynn killed the couple with murderous intent at the remote campsite they shared the following day.

The defence team argues they were "tragic, accidental" deaths set in motion by a dispute and two struggles between Mr Lynn and Mr Hill, the first involving one of Mr Lynn's guns and the second involving a knife.

The trial continues.