The Waikato Medical Officer of Health says holidaymakers must heed warnings not to eat shellfish due to the possibility of them being poisoned.
Health warnings are in place on both the west and east coasts of the North Island.
On the west coast the ban extends from Taranaki to Kaipara Harbour and on the east coast from Tairua to Whakatane Heads.
The toxins get into shellfish from naturally-occurring algae growing in sea water.
Dr Dell Hood says the recent illness of 27 people from shell fish poisoning in Tauranga and Rotorua, shows people are ignoring the warnings.
She says paralytic shellfish poisoning is one of the most serious toxins produced by marine algae and the severity of the recent illnesses is a wake-up call.
Dr Hood says all warnings should be taken very seriously.