New South Wales Health authorities say there has been a huge jump in the number of people suffering Ross River virus after being bitten by infected mosquitoes.

So far this year there have been 540 cases compared with 80 cases during the same period last year.

The Health Department's Director of Communicable Diseases, Dr Vicky Sheppeard, said that represented a seven-fold increase.

Most cases have been on the state's north coast, with about 300 cases on the mid and far north coast.

But she said there had also been up to 50 people diagnosed with the virus in Sydney and other instances in the Murrumbidgee area in the state's south.

Dr Sheppeard said the weather conditions were probably a factor in the sudden spike and there could be more cases to come.

"It's probably because it's been a fairly warm, wet summer," she said.

"We're also using a more sensitive trapping device for the mosquitoes so perhaps we're finding more virus but that doesn't explain why so many people are infected, so it is likely that there are more mosquitoes carrying the virus this year."

No treatment available for Ross River virus

Dr Sheppeard said there was no treatment for the condition.

"It's an unpleasant virus. It causes a fever, it can cause a rash and symptoms similar to arthritis in the joints," she said.

"It can last for weeks or even many months in some people, so that's a pretty unfortunate illness to have."

She has urged people to avoid being bitten in the first place.

"Over the Easter period, people are going on holidays and spending more time outdoors so they should use an effective insect repellent and cover up with long-sleeved light coloured clothing," she said.

Dr Sheppeard warned it was also possible that mosquitoes could carry viruses that are even more serious than Ross River, including the Kunjin virus and Murray Valley Encephalitis.

NSW Health said insect repellents should not be used on infants under three months of age.

Parents should instead use physical barriers such as netting on prams and cots and over play areas.