Michael McGrath murder trial: Christchurch river visited by murder-accused David Benbow 'highly suitable' for body disposal - expert

While police investigated David Benbow for murder, they were advised to tell the media they’d engaged an international expert in body disposal sites.

The theory was that if Benbow had disposed of Michael McGrath, it may prompt him to go and check the site. But neither a weapon nor a body were ever found.

A body disposal expert who studied McGrath's murder has given new information on the body disposal.

When McGrath went missing, body disposal and homicide expert Mark Harrison studied the case.

Harrison beamed into the Christchurch High Court from Australia as a Crown witness on Monday.

The former Australian police head of Intelligence told the jury if McGrath had taken his own life, he would likely have been found.

"The initial search actions, publicity and inquiries made by New Zealand police were likely and sufficient for an effective suicide search scenario to have discovered Mr McGrath if he had killed himself away from his home address," Harrison said.

Harrison told the court McGrath talked to his partner on the phone on May 21, then police visited Benbow two days later.

"This provides a substantial window of time in which to kill, transport, dispose, return and clean up the crime scene," Harrison said.

When police were investigating Benbow as a suspect, Harrison advised police to tell the media they had engaged an international body disposal expert because he said it may prompt Benbow to go back and check the site.

Police had a tracker on Benbow's car and in the days following that media release, he took two trips to an area around the Halswell River and adjacent wetlands.

"I am of the view that the Halswell River and adjacent wetland would have presented a highly suitable location to conceal and dispose of McGrath's body," Harrison said.

Benbow's lawyers put hours of questions to the witness around his credentials and relevance as an expert.

"Rather than an impartial expert here to assist the court, aren't you really just if you like an extension of the police who actually worked on the investigation?" defence counsellor Marc Corlett KC asked.

"No, I refute that totally," Harrison responded.

McGrath's body has never been found and Benbow has pleaded not guilty.

The trial is now in its fourth week.