The hospital opened on Tuesday in a Melbourne Showgrounds pavilion, less than three days after construction started.

Run by volunteer nurses, the hospital has capacity for about 30 patients and has so far rostered staff until Friday.

St John Ambulance Victoria received a call on Saturday after it was announced tower residents in North Melbourne and Flemington were put into hard lockdown.

"If they need care they'll come here instead of presenting themselves to an emergency department in the area," St John Ambulance operations manager Daniel Ciccosillo told AAP.

"It's just another layer of protection to keep people who might have COVID or who do have COVID in those towers away from the other population in the hospital system."

The lockdown of residents at the nearby towers was expected to end after five days, but some reports suggest it might extend to a fortnight.

Mr Ciccosillo said they were in contact with the health department and were able to respond to any needs beyond the original schedule.

The hospital operates from 9am to 9pm and has established three separate spaces for patients - a waiting area, one for those those needing attention and another for possible coronavirus cases.

A volunteer nurse at the hospital, Jarryd Redinger, said he wanted to help as much as he could after hearing about the situation at the towers.

He usually works at a hospital in the eastern suburbs, but will be at Flemington throughout the week.

"I wanted to assist to make sure they get the appropriate medical care," he said.

"I hope we can also assist the wider community and prevent the health system becoming overwhelmed."

All up, 69 positive cases have been detected at the public housing towers, including 13 new cases on Tuesday.

Australian Associated Press