Health warnings were issued after a thick haze from the bushfires in Victoria descended on Melbourne Tuesday morning, reducing the air quality in the Australian city to the "worst in the world".

Melbourne's air quality went down overnight to "hazardous" due to bushfire smoke.

Victoria's chief health officer Brett Sutton said, "I think overnight for Melbourne it did reach the worst in the world." Sutton said warmer temperatures would help lift "particulate matter" that had resulted in the poor air quality. He warned that people might experience a worsening cough and a dry nose and throat.

"For those vulnerable groups — over 65s, under 15s, pregnant women and people with existing lung/heart disease or diabetes — we are saying avoid exposure to the smoke by staying indoors and limiting physical activity," he said.

According to the Environmental Protection Authority's AirWatch website, the air quality in Melbourne was "very poor'' and visibility down to 900 metres in some parts of inner Melbourne Tuesday morning.

Sixteen fires were still burning and 1.4 million hectares have been destroyed across Victoria.

The entire state was choking under a blanket of bushfire smoke, including Geelong town, the Latrobe Valley, Central Victoria, Gippsland and the North Central regions, considered to have "hazardous" air quality at 7 am (local time).

In Melbourne's Central Business District (CBD), the smoke was measured at "hazardous" levels between 12 am and 4 am (local time).

While heart and respiratory patients were being told to keep safe, the healthy were being warned to stay inside.

ABC report said several outdoor suburban swimming pools were shut down and horse races at Werribee suburb were also cancelled.

Australian Open organisers also temporarily suspended qualifying matches and player practice sessions for Tuesday morning.

Sutton said people in East Gippsland and the state's north-east, where poor to hazardous air quality is likely to remain for some time, should try to let fresh air into their homes when possible.

"When conditions are good that's the time to open doors and windows to air your house, to get the smoke out and then when the air quality's poor, close doors and windows, stay inside, minimise your activity," he said.