Australia is on track to record its highest number of rubella cases in five years despite the country recently being declared 'free' of the contagious illness.

A spike in cases of the contagious viral illness rubella has been recorded just months after Australia was declared "rubella-free".

NSW Health on Tuesday said 13 cases had been reported nationwide in 2019, with seven identified in NSW.

Australia hasn't recorded more than 17 cases in a single year in any of the past five years.

The disease, also known as German measles, is spread through the air and causes a mild illness and symptoms such as a runny nose, rash and sore red eyes.

In pregnant women, it can cause a number of serious birth defects or miscarriage - though such incidents were seen locally fewer than 13 times in the 13 years to 2016.

The World Health Organisation in October 2018 verified Australia had eliminated the illness, meaning there was no ongoing local transmission.

NSW Health says the recent rise could be due to more sensitive tests and more doctors testing those suspected of having measles-like rashes.

Rubella cases have increased in the Western Pacific region, including Japan where more than 1100 cases have been reported in 2019.

"However, Japan has not been the source of infection of the recent cases identified in NSW," a NSW Health spokesman told AAP in a statement.

"As rubella is usually tested for at the same time as measles, this might also help explain why we have had more cases of rubella reported than usual."

Cases of rubella dropped significantly in the 1990s after the introduction of a second dose of the measles-mumps-rubella vaccine for school children.

Those born between 1966 and 1994 unsure if they have received both doses can access the vaccine for free at their GP.