Water supply staff shut off an Auckland city reservoir on Sunday afternoon after being told it contained cyanide.
But after a couple of hours, the water supply was cleared as safe.
Watercare Services Limited says Auckland City Council received a warning mid-afternoon that cyanide had been poured into the Hopetoun Street water reservoir near Karangahape Road.
The warning came from a group calling itself the United Freedom Fighters.
Watercare Services took the threat seriously and immediately isolated that reservoir from the rest of Auckland's reticulation.
During testing, the water supply was disrupted to Auckland's central business district, particularly Queen Street, Nelson Street, Hobson Street and Freeman's Bay.
But early on Sunday evening, the all clear was given.
Police are investigating.