The number of measles cases in Melbourne's north has climbed to 14 after the Health Department was notified of another five patients.
The department said the outbreak was in the Brunswick area and included two children who were pupils at Princess Hill Primary School.
A number of students at the primary school have been told to stay home for at least the next week because they are not fully immunised.
Victoria's acting Chief Health Officer, Dr Roscoe Taylor, said some teachers may also need to stay home.
"We appreciate that this situation will be disruptive for the school community but we must not put at risk those children who are not fully immunised," he said.
"Some teachers and support staff may also need to stay at home if they can't demonstrate their immunity."
Measles is a highly infectious disease with an incubation period of 18 days.
Anti-vaccination campaigners blamed for outbreak
Some doctors have blamed anti-vaccination campaigners for the outbreak.
Earlier this month four cases of measles were detected in the northern suburb of Brunswick, and yesterday a further five cases brought the total to nine.
The highly infectious viral disease can cause serious illness, particularly in young children and adults.
The initial four cases — a man and two women in their 20s and a woman in her 40s — all live in Brunswick or East Brunswick.
Of the recent five additional cases — three women and two men, all in their 20s and 30s — three of those live in Brunswick, one is from Preston and one is from Brunswick West.
Victorian health authorities expect the disease to spread and some doctors are blaming anti-vaccination campaigners for the outbreak.
Australian Medical Association Victoria president Dr Tony Bartone told AM "the fact that the immunity in the community is dropping, the fact that we've got dropping rates of the herd immunity in the community — that is clearly part and parcel, one of the reasons".
Vaccination advocate Dr John Cunningham agrees.
"I'd have to say the proof is in the pudding. Brunswick has the measles outbreak and currently the areas around it don't," Dr Cunningham said.
Last week the National Health Performance Authority revealed that about one in 10 Australian children were not fully immunised.
In Brunswick, the immunisation rate for children aged five is 90 per cent, 89 per cent for those aged two, and 94 per cent for one year olds.
The national aim is for a 95 per cent immunisation rate.
But Victoria's acting chief health officer Dr Roscoe Taylor said there was no evidence that anti-vaccination campaigners were to blame for the Brunswick outbreak.
"There's no real evidence. What we see in the particular Brunswick area is that the childhood vaccination coverage rates aren't that different from the state average, or indeed the Australian average," Dr Taylor said.
"So in terms of children, we've got good evidence that it's probably not about that, but we do not have good data on the vaccination coverage rates in older people like young adults, and so it's a bit hard to do more than conjecture about that."
Neighbouring residents worried as outbreak spreads
Regardless of the cause of the outbreak, Dr Taylor said the measles outbreak would spread, and expected more cases to be confirmed as early as today.
"We suspect there are several more probable cases in the pipeline. We will get some further confirmatory tests shortly," he said.
"Some of these people have to be treated in hospital."
Dr Taylor added that additional cases were more than likely to emerge in the coming days and weeks.
"I think it's highly possible that we shall see more in the coming weeks, and it really depends on what we call the level of 'herd immunity' in the population. But it's very likely that we will see more," he said.
Dr Tony Bartone is a GP in the Melbourne suburb of Niddrie.
He said his patients were raising concerns about what was happening just a few suburbs away.
"A lot of my patients are asking me about risks involved of travelling through Brunswick because it is a neighbouring suburb," Dr Bartone said.
"Brunswick is right in the epicentre, Brunswick is close to the inner CBD, and a very built up area."