The massive police probe into the disappearance of Christchurch builder Michael McGrath will focus on a Canterbury rubbish dump next week.

Police and family say they now have grave concerns for 49-year-old McGrath who was last seen at his Checketts Av home in the suburb of Halswell on May 21. He was reported missing two days later.

Detective Inspector Darryl Sweeney, who is leading the missing persons probe now entering its third week, said the search for McGrath has been comprehensive, on both land and water.

The Police National Dive Squad joined the operation this week and completed searches in the Halswell area. Sweeney said today they found little of interest.

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Updating the media at Halswell Quarry today, Sweeney said the search next week will be focusing on the Kate Valley landfill centre in Waipara, North Canterbury.

"We have information that does lead us to go to that location which might provide evidence as to Michael's whereabouts," Sweeney said.

Police last week confirmed that Christchurch Men's Prison Corrections officer David Benbow is "a person of interest who needs to be eliminated from the inquiry".

Sweeney today said Benbow still needs to be eliminated.

The prison guard's rural property just a few kilometres from McGrath's house has been the focus of a massive police investigation.

Police say the Benbow property and some motor vehicles will be handed back over the next 24 hours.

Sweeney said there are a number of persons of interest, coming to police daily, and they are speaking to them and eliminating them as they go.

There are no "suspects", Sweeney said.

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Asked to comment on rumours that McGrath had been in a relationship with a person of interest's partner, Sweeney said: "I can't speculate on that finer detail at present."

The police probe, called Operation Renovation, has more than 100 people involved across various phases, Sweeney said.

And it remains a missing persons inquiry.

"We're still looking for evidence of Michael and is he alive," Sweeney said.

"Police are committed to finding Michael and understanding the circumstances that led to his disappearance, which is out of character."

He confirmed that officers have been in the Little River area.

"We have had a look out there. We are tracking CCTV, generally around service station and major highways out of Christchurch."

ESR (the Institute of Environmental Science and Research) will be looking at what was found at the various scenes over a "number of days and weeks", he said.

Meanwhile, the McGrath family has now asked for privacy and requested that media please stop contacting them.

The inquiry team have received some good information from the public in relation to McGrath's disappearance, however information is still sought, police say.

Anyone with information in relation to his disappearance is asked to contact Christchurch Central Police Station on 03 363 7400.

Information can also be shared anonymously via Crimestoppers on 0800 555 111.