Toxic sea slugs which killed several dogs on the beaches of Auckland's North Shore over the last two winters are back, this time on Waiheke Island.

Auckland Council positively identified two of the slugs on rocks at Matiatia Bay on Waiheke Island last week.

Auckland Regional Public Health Service (ARPHS) has issued a warning that beachgoers should avoid touching or handling sea slugs at all beaches across the Auckland region.

"The small size of sea slugs means it's important for beach-goers to be mindful of little children and what they may pick up at the beach.

"We want people to enjoy our beautiful coastline but keep in mind this potential on-going risk to children and pets," ARPHS medical officer of health Dr Simon Baker said.

The ARPHS issued the following public health advice.

* Children and pets need to be supervised on all Auckland beaches, not just those bordering the Hauraki Gulf.

* Adults, children and pets should not eat anything found washed up on any beach.

* Parents need to be aware of where their children are swimming or playing, and what they are handling.

* Sea slugs on any beach must be avoided.

Anyone who finds a sea slug along the Auckland coastline should mark the spot, leave it alone and contact Auckland Council and ask for an environmental health officer, who will collect the slug.

Auckland Council marine scientist, Dr Jarrod Walker, said he was not expecting the high density of toxic sea slugs that occurred at Narrow Neck and Cheltenham beaches during the 2009-2010 season.

Auckland Council said it would continue monitoring beaches.