The granddaughter of a missing elderly Hamilton man has made a desperate plea to the public to help find the "kind hearted, gentle" 84-year-old.

People are scouring northeast Hamilton and a boat has been deployed along the Waikato River as the search for Raymond Stirling enters its fifth day.

Hamilton police and LandSar crews have extended their search area, and family have also mapped out a wider search area after they and authorities had thoroughly covered ground close to Stirling's Halcione Cl home, Marina Rameka told Newstalk ZB.

"We're remaining positive and we've mapped out a wider search area that search and rescue cannot completely cover. We've got already an amazing turnout from the community, who are already lining up outside with a desire to help us, which is incredible."

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They were also handing out flyers for volunteers to spread around the community, with family especially keen for the information to reach bus and taxi drivers, who may have seen Stirling.

Between 50 and 100 people were already helping, but more were welcome and could come to the family's base at in the garage of 29 Halcione Close.

She wanted to thank everyone who had offered to help, Rameka said. The family's main concern as that Stirling, who is described as "slim and frail", had hurt himself.

"He has a really big family who care about him immensely. We all just want him to come home safely."

Stirling was last seen on January 15. The Hamilton man has mild dementia and has been described as frail. His absence is causing extreme concern for police and his family.

Constable Sean Keeley said in a statement today the search would be expanded outside the suburb of Flagstaff, where the search had been focused for the past four days.

This morning some 40 New Zealand Land Search and Rescue volunteers, including groups from Auckland, Tauranga and Rotorua, have resumed the search.

The Hamilton police boat would also be out on the Waikato River this morning.

Keeley said Stirling was known to hitch a ride from time to time.

"We know it's possible he may have hitchhiked somewhere, including outside of Hamilton."

He said if anyone had given a ride to someone matching Stirling's description to contact police on 111.

Keeley also urged residents to keep an eye out for the elderly man and contact police if they saw him.

"We urge people in Hamilton to again check their backyards, sheds and other outbuildings in case Mr Stirling has taken shelter there," he said.

"We are working hard to locate Raymond and even the smallest piece of information could help, so please don't hesitate to contact us."