Affected areas include Greymouth and the suburbs of Cobden, Blaketown, Karoro and South Beach, plus Boddytown, Paroa, Kaiata, Dobson, Taylorville, Stillwater, Runanga, Dunollie, Coal Creek and Rapahoe.
The notice was enacted at 10am and remains in place until further notice.
"While the source of the contamination is still under investigation, council is taking a proactive approach to safeguard public health," Grey District Council chief executive Joanne Soderlund said.
"As part of our standard procedures, the water supply is tested regularly and in this instance, sampling results have indicated a need for further investigation and temporary protective measures. The council has immediately commenced a programme of daily sampling and monitoring to confirm water quality."
John Paul II High School closed at 12.30pm due to the water situation.
Principal Renee Hutchinson said although that meant children were out of school before school buses were running, they had access to clean drinking water in town.
The school would be open today and tomorrow, but students had to bring their own water.
A post on the John Paul II High School social media page originally said the school had been advised of an E. coli outbreak but the council quickly asked the school to remove any reference to the bacteria.
A council spokeswoman said it had not yet been confirmed what contaminant was found.
"Nothing will be confirmed until testing comes back. We just want it out there as far and wide as we can."
Greymouth High School and St Patrick’s Primary School both brought in bottled water and stayed open yesterday.
Grey High School principal Samantha Mortimer said yesterday the board of trustees would decide what to do for the next few days.
The "boil water" notice affected things such as hand washing too, so the school was assessing sanitiser stocks, she said.
If the school did close, students would move to home learning.
Schools had secured bottled water for the Grey District Athletes event under way yesterday at Anzac Park. Some schools had asked parents to bring water down to the park.
Following national drinking water standards, three consecutive days of clear test results are required before the notice can be lifted.
Residents in all areas connected to the town supply are advised to boil all water used for drinking, food preparation, brushing teeth and making infant formula until further notice.
The Grey District Council reinforced the warning with an automated alarm over cellphones early yesterday afternoon. — Greymouth Star
By Meg Fulford and Laura Mills
