Three new cases of measles over the past four days in Victoria are believed to be linked, with health officials warning of an increase in the risk of a “significant” outbreak.

The new cases raised concerns that there were now “multiple undetected cases in the community potentially spreading the infection,” Victoria’s acting chief health officer Finn Romanes said.

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Dr Romanes said the three include a young woman whose case was made public on Friday.

She had been in Shepparton, at Southern Cross Station and Melbourne Airport between June 21-25 when she was infectious.

She was also in Queensland.

About 91 per cent of children aged 24 to 27 months have been immunised against measles. Photo Getty.

Sufferers may still be in incubation phase

All of those contracting the illness were between 18 and 30 and had not travelled overseas.

However it can be brought in by travellers from overseas.

There is no direct connection between the three cases, but investigations suggested the measles had possibly spread to Geelong, the Surf Coast, the western, north-western suburbs of Melbourne and the inner city.

“Our concern is that there was a person or persons who probably had travelled overseas, and have since unknowingly passed on measles to these three people in the western suburbs and Barwon area — and there may be more,” Dr Romanes said.

“There is now the potential for these three people and anyone else who has been infected to pass on the disease and create a significant outbreak.

“Anyone unwell with a fever and rash who is not fully vaccinated for measles should ring ahead to their doctor or hospital and alert them that they may have measles.”

Measles has an incubation period of about 18 days and the people were probably exposed through contact with the three cases from mid-to late June.

“So illness acquired from contact could still be coming through, and cases could still remain infectious for many days,” Dr Romanes said.

Measles, a highly infectious viral disease, is now uncommon in Australia because of the widespread use of the measles vaccine.