The West Coast District Health Board is working to discover what caused a mystery illness outbreak at a primary school.

Sixty-six Westport North School students - about a third of the school - were away sick yesterday.

Principal Cath O'Loughlin says they have decided to close the school for the next three days to disinfect and clean all surfaces.

“We will be disinfecting and cleaning all surfaces in the school to stop the unknown illness spreading any further.”

She says they will make a call on Thursday about whether they will reopen on Friday.

Medical Officer of Health Dr Ramon Pink says they've taken samples and will be carrying out tests - but they're not sure what the illness is.

But he says flu-like illnesses have been circling on the West Coast.

Pink also says gastroenteritis is an issue and people who have it should stay away from school for 48 hours after their symptoms stop.