An expert defence witness in the trial of two men accused of killing a Wellington journalist says based on CCTV footage there was no time for one of them to be part of the attack.

Radio New Zealand journalist Phillip Cottrell was rushed to hospital after being left with broken bones and a shattered skull in an attack on a central Wellington footpath early on December 10 last year.

He died in hospital the following day.

Nicho Waipuka, 20, and Manuel Robinson, 18, are on trial at the High Court at Wellington accused of his murder, which they deny.

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Business security consultant David Horsburgh has studied CCTV footage from a Telecom building on Boulcott Street.

He said it showed Mr Cottrell at 5.37.46am and he calculated that he was walking at 1.46m/sec.

Mr Horsburgh said the same camera showed Robinson moving in the opposite direction on the other side of the road about 28 seconds later.

It would have taken about 22 seconds to get from the attack site to the Telecom building, he said.

There would be only 5.41 seconds for Robinson to participate in an assault then run back to view in the Telecom CCTV footage, he said.

Crown prosecutor Tom Gilbert said Mr Horsburgh had incorrectly rounded down his walking pace calculation, and in fact Mr Cottrell could have got to the attack site earlier than Mr Horsburgh estimated.

Mr Horsburgh said the difference would be "fractions of a second".

But he conceded that if Mr Cottrell had increased his walking pace down the slope to where he was attacked, he could have got there sooner than his prediction.

"I do not know if he increased or decreased his speed, sir."

Mr Horsburgh studied pedestrians for three periods during the course of one day to establish the line Mr Cottrell would have walked along Boulcott Street.

Mr Gilbert asked how many people in his sample group were about to be attacked, were being yelled obscenities at and had been beaten up and left for dead.

"None sir," Mr Horsburgh replied to each question.

"Then your sample adds nothing to how someone might react in those circumstances," Mr Gilbert said.

Mr Gilbert said earlier witnesses had given evidence that Mr Cottrell would have avoided confrontation at any cost.

Mr Horsburgh said it would take an "enormous increase in pace" for there to be time for Robinson to cross the road, assault Mr Cottrell, cross back and be filmed by CCTV footage 28 seconds later.

The trial continues.