The number of Aucklanders infected with the stomach bug Norovirus more than doubled over one day.
On Sunday, 20 staff members at South Auckland's Middlemore Hospital and 19 members of the public had fallen ill.
There were 18 confirmed cases on Saturday.
Middlemore Hospital spokesperson Lauren Young says clinical staff believe the virus is coming in with patients from the community.
She says they are taking every precaution to stop it spreading within the hospital and urging all visitors to take extra care with washing their hands.