'Time is critical': Search for Sydney man missing in dense bushland enters fourth day

The desperate search for a man missing in dense Sydney bushland will enter its fourth day today.

Ronald Weaver, who suffers from Alzheimer's and other medical conditions, was last seen on Woolcott Avenue in Wahroonga at 10am on Tuesday.

Up to 100 emergency workers are involved in the search, which is focused on bushland at Cliff Oval in the suburb.

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The wife of the 79-year-old issued an emotional plea for help yesterday.

Weaver's wife Hannah urged locals to "remember his face" from police photos.

"He's a good person, even though he has Alzheimer's I believe he still deserves quality of life," she said.

"Please, please help to locate Ron.

"He remembers his name, even though he doesn't remember his date of birth."

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Acting Superintendent Carlene Mahoney asked locals to check around their houses as the NSW Police dog, mounted and trail bike units, SES, PolAir, NSW Rural Fire Service and National Parks and Wildlife Service staff all join the search.

"We urge people in the area to look in their backyards, sheds, driveway and you may find Ron has taken shelter from the heat over the past few days," she said.

"Time is critical.

"We're urging the public to search their areas and if you're out on the trails please look out for Ronald."

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Police are also door-knocking local homes and asking people to check any CCTV they have.

A targeted SMS message has also been sent.

Weaver is described as Caucasian, about 155cm to 160cm tall, thin with grey hair which is bald on top.

He was last seen wearing a cream-coloured shirt with a Sapporo beer logo, grey shorts and striped socks.