Paramedics in Rotorua have treated 15 people who had used synthetic drugs in just 24 hours.
Ten deaths have been linked to synthetic drugs in Auckland in the past few weeks.
St John said its paramedics in Rotorua responded to 11 incidents involving 15 people between 1.40pm on Thursday afternoon and 10.30am Friday.
Police the synthetic substances were highly dangerous and they were urging anyone using them to stop because they were putting their life in danger.
Anyone with information about the drugs was urged to contact police.
On Thursday, police announced they had seized a large quantity of psychoactive substances that could have made $1.5 million worth of synthetic drugs.
They said the raw materials could have made 150 kilograms of the synthetic product.
Customs said the seizures at the border were an indication of the potentially lethal new drugs that people are trying to bring into the country.