Police are expanding their search for a 73-year-old Auckland woman with Alzheimer's, four days after she went missing on the North Shore.

Pat Wearn was last seen leaving her Torbay home around lunch time on Monday to go for a walk.

More than 90 people are involved in the search for Wearn, including search volunteers, Police officers and Defence Force personnel.

Crews worked until nightfall last night, and are continuing at daybreak with an expanded search of bush areas and a door-to-door area canvass.

Police have received advice that Wearn will be dehydrated and disoriented, but is physically fit and healthy and should survive.