Bay of Plenty Regional Council is trying to avert an algal bloom on Lake Rotoitoi.
The council said bacteria growth in Okawa Bay has reached a point where it could pose a risk to human health.
General manager for natural resources Warwick Murray said the area will be sprayed with a chemical compound, which should prevent an algal bloom.
He said a similar attempt in 2011 was successful in preventing a bloom developing.
Mr Murray said people have been notified about the spray and signs are posted at the Okawa Bay boat ramp warning that it's not safe to swim in the lake while the spraying is done on Sunday.