Problems with Dannevirke's water supply forced the closure of some Dannevirke schools, while others were confident they could manage the risk and opened.
The Tararua District Council issued a precautionary boil water notice for the Dannevirke water supply on Thursday due to an equipment failure.
"There's been a technical failure at our water treatment plant and Scada and UV equipment are not talking to each other," Mark Maxwell, the council's economic development and communications manager said.
"We are drawing water from the river and it's still going through part of the treatment process, but not all, and it is being chlorinated."
Maxwell said while water was being drawn directly from the river intake and still being chlorinated, it would require boiling before consumption.
"It could be cloudy or milky and, once the problems are fixed, it might take up to 24 hours for that cloudiness to disappear."
Dannevirke South and Huia Range schools and Totara College made the decision to close on Friday but Dannevirke High School and St Joseph's School remained open.
Debbie Max, principal of Totara College, said her school had been told too late of the option to have a water tanker.
"That notice came after we'd informed everyone we were closing," she said.
The Tararua District Council had a water tanker in place at the high school and students were told to take water bottles to fill.
"All schools follow the appropriate processes themselves," Maxwell said.
Sheryl Berry, administrator at St Joseph's School, said they were "quite confident" they could manage the situation.
"We have taped off all our drinking fountains and children have been asked to bring their own bottled water.
''We will also have spare water bottles for those who don't," she said.
Maxwell said the council was working according to its water safety programme which is aligned to Ministry of Health requirements: "This is not an E. coli outbreak. It's just that the quality of the water has been reduced and we have to manage any risk.
''The boil-water notice has been put in place as a precaution and water going through the treatment plant is being chlorine dosed."
Council was hoping to have the problems sorted over the weekend.
Boil your water
■Until notified, Dannevirke residents are advised to boil water before using it for drinking (including making of sachet juice/drinks), making ice, food preparation, brushing teeth and preparing infant/toddler formula.
■Electric jugs with a cut-off switch can be used as long as they are full - allow the water to come to the boil and wait for it to switch off (do not hold the switch down to increase the boiling time). Water can also be placed in a clean, metal pan and brought to a rolling boil for one minute. Boiled water should be covered and allowed to cool in the same container.