Auckland City Council is warning residents to supervise their children and dogs across eastern beaches from Okahu Bay to Kohimarama.
The warning follows the discovery of more toxic sea slugs. Some have been collected from Mission Bay for the first time, while another was found in a rock pool close to Kelly Tarlton's Underwater World.
The council says it is putting up further signs warning beach users about the slugs and will be distributing leaflets to local residents.
Several dogs became ill, and five died, after eating the grey sea slugs, which contain tetrodotoxin, almost a year ago.
Further sightings should be reported immediately to the council.
Gone by October, hopefully
The technical manager at the Cawthron Research Institute, Paul McNabb, says the slugs' resurgence is due to their life cycle.
They are likely to be in their highest numbers on Auckland's beaches about now, he says, as their breeding season comes to an end.
Mr McNabb says that if last year's pattern repeats, most of the slugs will be off the shores after October.