Several dogs became sick, with some dying, after eating the grey sea slugs, containing tetrodotoxin, almost a year ago.
Auckland Regional Council says divers have found a total of about 100 sea slugs at seven of the nine beaches they looked at.
Narrow Neck Beach on the North Shore had the highest number, with 48 slugs found there.
The Regional Public Health Service says beach-goers should keep a close eye on children and pets while visiting beaches, to avoid the risk of poisoning.
It says people should phone their local council if they see a sea slug and ask for an Environmental Health Officer to collect it.