Queenstown. Photo: RNZ / Marika Khabazi
A senior health official believes there are more than 31 cases of cryptosporidium in Queenstown, they have just not been reported.
Most infections have been in the Queenstown CBD area and those sick were mostly aged between 15 and 39 years old, with one case under 10.
As of Monday, there were 31 confirmed cases, seven probable cases, and eight under investigation.
Health officials have still been unable to identify the source of the outbreak but are considering all possibilities, they said in a statement.
The Southern Medical Officer of Health Susan Jack told Morning Report that most of the cases were unlinked.
"The more people who are tested ... the more information we'll have to put this puzzle together" - Southern Medical Officer of Health Susan Jack
"I'm sure that there's more than the cases that we've got but we are urging people, if you do have symptoms or have had recent symptoms, please can you call your GP.
"The more people who are tested and who we can interview about what's going on, the more information we'll have to put this puzzle together."
Officials had been looking at water supply as a potential source of the gastro outbreak, resulting in a boil-water notice.
But Dr Jack said they were widening the scope, looking also at food or drink products.
"You may get some person to person spread but we're kind of focussing on, 'Is there a particular food that is in common or drink product?'."
They were also investigating any links in the CBD area, she said.
Dr Jack said the boil-water notice would only be lifted if either an alternative source for the outbreak can be established or if it is confirmed that water is the problem, clear steps are taken to mitigate it.
She said health officials, along with council and MPI had been working throughout the weekend and would continue to move as fast as possible to resolve the situation.