Queenstown's cryptosporidiosis outbreak continues to grow with 40 people now confirmed to have been infected as a result of the cryptosporidium parasite.
There are also another 10 probable cases and another eight under investigation.
But the source of the outbreak is yet to be identified, and a boil water notice is still in place.
Te Whatu Ora Southern Medical Officer of health Dr Susan Jack told Checkpoint the source could include drinking water, recreational water, food or drinks.
"We're still actively searching for the source ... so far nothing has come up as a red flag from our interviews with cases."
Authorities were now also finding person-to-person spread.
"So handwashing with soap and water is absolutely critically, and especially drying your hands before you continue.
'Unfortunately, alcohol sanitisers are not effective for crypto, and then our other big message is not to go swimming until you've had no symptoms for at least two weeks."
Read Queenstown Lakes District Council's notice about cryptosporidium infection and keeping safe here.
The illness can cause severe diarrhoea, nausea, vomiting, stomach cramps and pain, headache, lack of appetite, fever and tiredness, the government's HealthEd website said. Babies, the elderly and immune compromised are particularly vulnerable if they get infected.
Anyone with those symptoms has been asked not to go to school or work, as the parasite can be spread easily, and to contact a doctor both for advice and because public health officials are carefully monitoring the situation.
Queenstown has had a rough week, after a state of emergency was declared last week due to slips and storms (- since lifted), as well as the boil water notice and outbreak. Photo: RNZ / Marika Khabazi
A notice to boil all water used for food, drink or dishwashing is still in place for all properties and businesses on the Queenstown water supply. Queenstown Lakes District Council said water needs to be boiled for more than one minute. It warned that water to be made into ice, filtered water and water heated in zips all still need to be boiled for 1 minute additional to those treatments.
Dr Jack said most people who had been infected were aged between 15- and 39-years-old - which was about what they would expect as a representation of the population's demographics, and most had been in Queenstown from mid-August onwards.
"It still could be the drinking water supply, we're not ruling that out, but it's looking like it's what we called point source outbreak - so something happened, there was a contamination event, and that could have been in the water, or through food or beverage or something else.
"Our next steps is with help from ESR [ the Institute of Environmental Science and Research] we've developed a new more detailed questionnaire, so we'll be going through that with the 40 existing cases and all new cases.
"Then if there's a red flag such as a specific food or beverage or location we'll direct our efforts toward that."
A pool at Alpine Aqualand had been closed, as cryptosporidiosis infections could be caught from swimming pools - but Jack said it was able to reopen after it had been checked over: "They've looked at what they're doing for filtration, and feel that it's adequate".
