Six people in Melbourne have been diagnosed with Ross River virus, with heath authorities saying it could be the first time it has been detected in the city.

The cases have been detected around Frankston and Casey, in Melbourne's south-east.

There has been a jump in the number of cases in northern Victoria this summer, but the Health Department said this could be the first time the virus has been confirmed in the metropolitan area.

More than 800 case were detected in the north of the state this year. The average is usually about 20.

The virus causes painful swelling of the joints — particularly hands and feet — fever, fatigue and sometimes a rash. Nausea, headaches, backaches and muscle aches are also common.

Symptoms usually come on between two and 14 days after being bitten by an infected mosquito and disappear within six weeks.

But some people can have ongoing joint pains, depression and fatigue for many months.

Cover up body, clean yard to ward off mozzies

Health Minister Jill Hennessey said flooding in northern parts of the state last year made for perfect conditions for the disease.

"It's not a fatal illness, but it's a really uncomfortable one to have," she said.

"Part of the problem with the ongoing floods was there was a lot of stagnant water, that created ideal conditions for mosquitoes to breed.

"We know there's been a couple of hot days, people will be sleeping with their doors open, having their kids in makeshift swimming pools.

"It's really important to make sure if you don't have flyscreens on your windows, make sure you're wearing mosquito repellent, dressing in long-sleeved cotton materials.

"And this weekend get out and remove all the stagnant water that may be around the house."

Chief health officer Professor Charles Guest said the virus was not common around Melbourne.

"We don't expect to see cases in metropolitan areas, but we know there are a lot of mosquitoes, so it's not surprising," he said.

"Bare in mind, many people who are infected by Ross River virus don't show any symptoms at all."