The bushfire smoke blanketing Melbourne is expected to persist for at least the first half of the week, as air pollution in the city worsens to "very poor" levels.

Key points: Air quality in Melbourne and Geelong is forecast to drop to very poor by 5:00pm

Air quality in Melbourne and Geelong is forecast to drop to very poor by 5:00pm Elderly people and pregnant women are urged to stay inside or wear masks

Elderly people and pregnant women are urged to stay inside or wear masks Homes should be sealed up, experts say

The smoke is also affecting air quality in Geelong, which was upgraded to "hazardous" levels on Monday afternoon, but by evening it had improved to "poor".

The North East, Gippsland, Alpine and Latrobe Valley regions are also affected, according to monitoring by the Environment Protection Authority (EPA).

Though some of the smoke has come from fires in Victoria's north-east and East Gippsland, some of it has actually drifted across from Tasmania, EPA chief environmental scientist Andrea Hinwood said.

"We're not expecting this to change until Wednesday," Dr Hinwood said.

People may experience coughing and irritation of the nose, throat and eyes over the next two days from smoke. ( ABC News: Gemma Hall )

"We might see some light relief [on Tuesday] but we might not."

Dr Hinwood said rain had unfortunately "done almost nothing".

"Drenching rain would be terrific because it would knock it out. But the current amount is insufficient and there's too much smoke."

The smoke has even prompted tennis great Novak Djokovic to suggest Australian Open organisers consider delaying this year's event if the problem persists.

Navy, Air Force and Army crews are on standby as smoke blankets the RAAF Base in Sale. ( ABC Gippsland: Sarah Maunder )

'A threat to life for some'

Victoria's chief health officer Brett Sutton said people may experience symptoms such as coughing and irritation of the nose, throat and eyes.

Vulnerable people — such as those aged under 14 or over 65, those with diabetes or heart/lung diseases and pregnant women, children 14 and under, and people over 65 years old — have been urged to minimise smoke exposure by staying indoors.

If they need to be outside, they should wear P2 or N25 masks, Dr Sutton said.

"We know it's a threat to life for some people."

In Geelong, the air quality deteriorated to "very poor" levels making it difficult to see the city from the waterfront. ( ABC News: Nicole Mills )

Dr Sutton said asthmatics should put their asthma management plan into action, and those who did not have a plan should see a doctor to get one.

He said symptoms could also appear in people with undiagnosed asthma.

"People with ongoing cough, wheezing, tightness of the chest, may have asthma and should see a GP," he said.

He said smelling smoke inside was a sign of poor insulation in Australian homes and an indication of the high smoke levels in the city.

Dr Hinwood advised people to "close everything up" in their homes and seal their doors.

"Now is a good time to do that because this will persist for the next few days," she said.

"Some homes are terribly leaky and will be problematic. Go to the library or shopping centre to take a break in these conditions."

The EPA is advising people stay indoors and close up their homes to keep air quality inside at its highest. ( ABC News: Cathy Jacobs )

Should you wear a face mask?

The Federal Government is releasing 450,000 P2 face masks from a national stockpile to provide to at-risk Victorians and frontline workers.

Dr Sutton said face masks were "not a cure-all", but he said P2 masks could act as additional protection for vulnerable people forced to be outside.

"But masks can give you a false sense of security," he said.

"It doesn't mean it's okay to go outside. They have to be fitted properly and when they are fitted it can make it difficult to breathe.

"Surgical masks, bandanas are not going to do the job."

Melbourne's air quality is not forecast to improve before Wednesday. ( ABC News: Gemma Hall )

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