It all started with five students who caught measles on an Emirates flight from Singapore in January.

Those five cases quickly became 20, and before long the disease had spread to Hawkes Bay, where a further 24 people became ill.

Now, 10 months later, the highly contagious disease has infected nearly 300 other people around the country - 44 of whom were so ill they needed hospital treatment.

"It's a highly infectious virus, measles, it's really easy to catch if you are not immune," said Auckland's medical officer of health, Dr Richard Hoskins.

Dr Hoskins does not think this outbreak will come anywhere near the 1997 epidemic, which affected more than 2000 people, but he says there is little more authorities can do to slow the spread of the disease.

"We are at the stage now where what we are doing with quarantine and isolation is only going to have a small effect.

"It's people understanding the significance and making sure they are up to date with their immunisation that's the important thing."

He said measles was so contagious that people could easily catch it just by being in the same space as someone who was infected.

Not all of the eight people who contracted measles on the Emirates flight were sitting together.

"Some of them were sitting 11 rows and a whole section of the aircraft away," Dr Hoskins said.

After that initial outbreak, things quietened down until May, when a person associated with Oratia District School in West Auckland became sick.

That person spent a morning at the school, resulting in transmission to seven others.

Then a few other pupils and a "lot of other kids from the extended families" of those initial cases also became sick.

"That's a community where immunisation levels are lower than other parts of Auckland ... If it hadn't got in there it probably would have fizzled out," Dr Hoskins said.

From there, the disease reached the Ranui Baptist Church, where several people "had decided not to have their kids immunised".

Three cases were diagnosed and then two weeks later there were 14 more - but many of the parents stood by their decision not to immunise.

"I spoke to some parents who were absolutely convinced that they had made and were making the right decisions," Dr Hoskins said.

The most recent increase in cases has been centred on Auckland's inner suburbs such as Parnell, Ponsonby, Herne Bay and the CBD, affecting young people in their 20s and 30s.

Dr Hoskins admits that immunisation is not 100 per cent foolproof.

A small number of immunised people had still caught the disease, but the vast majority of those who had become sick - about 80 per cent - were not immunised.

By Elizabeth Binning Email Elizabeth