About 40 patients have become ill with a stomach bug at several Auckland health facilities.

Patients at North Shore Hospital, forensic psychiatric unit the Mason Clinic and Taharoto Mental Health Unit.

Waitemata DHB staff were alerted to the bug on Sunday evening after a number of patients became ill with vomiting and diarrhoea.

An immediate investigation is underway and staff are working closely with other agencies, including Auckland Regional Public Health Service, to identify the cause of the illness, chief medical officer Andrew Brant said.

The illness is thought to be a 24-hour bug and is not linked to the Listeria infection reported in Hawke's Bay last week.

Brant said infection control measures were being deployed as appropriate.

''Waitemata DHB does not use any of the meat products identified as part of that recall. In addition, the symptoms and the timeframe do not suggest any connection to Listeria.''

Patients discharged from any of the named facilities over the past couple of days who are experiencing vomiting and diarrhoea are advised to see their GP if symptoms persist.

- © Fairfax NZ News