Cell-tower data presented to a Dunedin court has raised questions over the location of a 21-year-old cleaner accused at the time of an alleged murder.

Alexander James William Merritt is before the High Court and denies murdering work colleague Karin Ann Ross (51) early on December 2 last year.

Merritt told police he had returned after work to his Kaikorai home about 9.30pm on December 1, and had remained there until the following morning.

But the Crown alleges that the Spotless Cleaning Services employee actually armed himself with a hammer, left the Nairn St property in the early hours of December 2, and travelled back to the work premises on Strathallan St - about 10 minutes drive through town.


Once there, he allegedly battered Ms Ross, his supervisor, leaving her to die in a pool of blood, before returning home. Her body was found at 2am that day.

Spark employee Lawrence Watson today gave technical evidence about cellphone data obtained from the defendant's Samsung device.

At 12.35am on December 2, Merritt's phone connected to the Halfway Bush cell site - a tower only half a kilometre from his home.

Mr Watson conducted tests two weeks after Ms Ross's death and found that site provided the strongest signal from Nairn St.

However, at 1.34am - when Merritt is adamant he was home - his phone pinged off the Dunedin North tower located on the corner of Hanover St and Great King St.

After driving three different routes from Strathallan St to the Merritt family home and monitoring two phones' activities, Mr Watson concluded the only way the phone could have connected with that cell site was if it was located on Highgate or on the city side of that road.

Nairn St is on the other side of Highgate.

The witness told the court radio waves which connected phones with cell towers could bend slightly.


But for Merritt's phone to use the Dunedin North cell site from his house, Mr Watson said the waves would need to bend almost 90 degrees, which was "almost unheard of".

"Our experience just doesn't support that," he said.

Under cross-examination from defence lawyer Alastair Logan, Mr Watson said he had been instructed by police to drive the three different routes while testing cell-tower coverage.

He accepted he had not used any back roads and had used a slightly different Samsung than the one Merritt owned at the time.

The court was also presented with information regarding cellphone usage on Merritt's device.

Mr Watson said records showed it was using data for more than 40 minutes from 1.37am on December 2 and for another session of 45 minutes starting at 5.34am that day.

It was impossible to say exactly what activity the phone was undertaking, he said.

The Crown case was expected to conclude today before the defence decides whether or not to call witnesses.