Health authorities are warning of the risk of a mass-outbreak of mosquito-borne viruses in flood-beleaguered parts of NSW.

An unprecedented plague of disease-laden mosquitoes in the NSW central-west has health authorities sounding the alarm.

There has been an almost 100-fold increase in mosquitoes in the regions near Forbes, Leeton, Griffith and Moama, with an extraordinary number mozzies carrying viruses according to tests by NSW Health.

"I am expecting to see one of the largest outbreaks of mosquito-borne disease ever recorded for inland areas of New South Wales," NSW Health scientist Stephen Doggett said on Monday.

Some of the mosquitoes are carrying Ross River and Barmah Forest viruses, which can cripple sufferers for months.

"They can cause persistent and debilitating symptoms such as joint aches and pains, fever, chills, headache and sometimes a rash," Mr Doggett said.

He also warned it is possible some of the mosquitoes may be carrying the potentially fatal Murray Valley Encephalitis or the Kunjin virus, which can cause severe brain infection.

There are no specific treatments for any of these viruses.

The recent flooding in the area is behind the spike in the numbers and the mosquitoes are picking up the viruses from animals, NSW Health said.

There have been more mosquitoes detected with Ross River than at the same time in the 1998-1999 summer, in which a record number of people infected.

Only once before was the Barmah Forest virus detected so early in the year, during a summer where there was more than four times the normal number of infections.

NSW Health is asking residents to take precautions to avoid being bitten, such as using mosquito repellent and covering up.