The number of confirmed cryptosporidium cases in Queenstown has risen to 31, but there are likely more in the resort town.
Southern Medical Officer of Health Susan Jack said that as at 10am today, there were also seven probable cases, and eight under investigation.
She said a source of infection still had not been identified.
Dr Jack had earlier told RNZ she believed there were more than the 30 cases that had been reported to yesterday.
Most infections had been in the Queenstown CBD area and those sick were mostly aged between 15 and 39 years old, with one case under 10.
Dr Jack told Morning Report that most of the cases were unlinked.
"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 focusing 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 had been working with with council and MPI throughout the weekend and would continue to move as fast as possible to resolve the situation.
