Almost 200 people gathered at Eureka Stockade Gardens for a morning briefing on February 24 before heading out in groups to search forest in Mount Clear.

It has been three weeks since Ms Murphy left her Eureka Street home for a run and did not return.

Detective Superintendent Mark Hatt from Crime Command on Friday said police were "very doubtful" that Ms Murphy was alive.

Investigators have been reviewing about 12,000 hours of CCTV footage and following up more than 500 separate pieces of information in the search for Ms Murphy.

It is believed she reached the Mount Clear area about an hour after she left home about 7am on February 4.

This bushland area was where community volunteers were setting out to search on February 24 and expert skills were on hand.

READ MORE: Timeline of Samantha Murphy disapparance

David Tong has 26 years' experience in the Australian Defence Forces and a further 15 years in the Queensland State Emergency Service. He has been involved in land searches for missing persons that have lasted hours to searches in territory, such as Moreton Island, that have lasted for days.

In between his work and study, he simply wanted to come out and help.

Bacchus Marsh-based Kerry Toner is a member of Australian Mine Explorers, a group best known from the television show Mine Shaft Explorers.

He is used to areas such as the old pine plantation near Mount Clear but he was not so familiar with the bushlands groups were to search on Saturday.

Mr Toner and his wife were setting out with prospectors, who were using their metal detectors, in a bid to offer his expertise about old mines. He said a lot of old shafts in the area were reported to be opening up with the weather - safety remained a high priority.

"We're up here to help the family and see if we can find anything," Mr Toner said. "We've been following since the start but circumstances change...We have a few friends up here coming."

Volunteer Natalie Squire, who has been part of the search organising team, said the numbers in the community turn out was what they had been "hoping for".

Ms Squire said missing persons was a community issue and organisers hoped for a resolution for Ms Murphy's family because ambiguous loss was extremely traumatic.

"Personally, I'm really overwhelmed and so grateful for the community support," Ms Squire said. "...Seeing the sea of hi-vis is incredible."

Ms Squire is a mum, a similar age to Ms Murphy and a theatre and school mum, like Ms Murphy.

She has been out walking in earlier search efforts and was keen to do what she could to help the community search on Saturday.

Similarly Simon Tait said he felt parallels to Ms Murphy, especially being a parent. He lives in Melbourne but grew up in Ballarat's east.

"My motivation was to try and help, one way or another," Mr Tait said. "I feel if you can help, you should and try to give some answers for the family.

"It's great to see so many people give a damn."

The search had about a dozens community business sponsors, including a sausage sizzle at a base camp for volunteers.

While children were keen to be involved in the search, organisers urged for them to remain helping volunteers at the base camp.