They do the "fish dance", the "funky chicken" or "just drop", say Auckland's homeless who have witnessed a sudden surge in a deadly new form of synthetic cannabis in their community.

Seven people have died in the past month after using an as-yet unidentified form of the drug, with ambulance staff seeing up to 20 users a day suffering its life-threatening effects.

An Auckland District Health Board spokeswoman said "numerous" people were admitted on Friday night and Saturday morning as a result of the drug.

Auckland City Mission also reported spike in aggressive behaviour and admissions to its detox unit in the last few months.

"We're definitely seeing an increase in both aggression and violent behaviour, that's probably what's most evident to us," Auckland City Mission chief executive Chris Farrelly said.

This included clients the mission had otherwise known as quiet and non-aggressive.

"We suspect is it's been laced with other stuff; other very, very damaging stuff," he said.

Furthermore, it was very difficult to detox someone off an unknown substance.

On central Auckland's Victoria St yesterday, it did not take long to find numerous regular users.

"It's really, really super strong and it's killing a few people out here," said one homeless man.

"I've seen them fish dancing on the ground, like fish without water, it's crazy."

The man, himself, had such a seizure on Thursday.

"I never, ever want to experience it again. You're just in a blackout and it's just like you're in another dimension, in another world, and when you wake up you wouldn't even know what happened.

"If I keep this up I'll have another one and I won't even get up, you know, forever," he said.

A group sitting outside a supermarket were trying to get enough money together to score.

"I just get it from dealers ... [they're] all over the place," said a man called Gordon.

The drug came in little zip-lock bags and generally cost about $20 for two or three joints worth.

"It helps me help have a good sleep, eh, when I'm trying to stay warm."

He used to smoke cannabis but had recently changed to synthetics.

"You can still buy [cannabis] I suppose but ... I find this is better... better wastage off it."

He'd been arrested as a result of his behaviour while high on drug and regularly saw others having seizures.

Another woman, Puawai, said many of those she knew used it every day.

"They find that synthetic was a cheaper buzz, the rush was a bit more abusing than other drugs.

"[They get] real intoxicated looks and actions, slowing their speech."

Another woman begging for money on Queen St, who said she did not use synthetics but was an alcoholic, was very angry to see the harm the drug was causing.

"I've actually seen people go down on the streets doing the funky chicken, that's what I call it," she said.

"It depends on whether or not the people can handle it and if they can't, they just drop."

The drug had only started circulating recently and people were buying it instead of other drugs.

The woman, who has been homeless a year and a half, said she knew one of those who had died, on Fort St, because of the synthetics.

"I keep saying to the young ones and everything, 'Don't'.

"It's horrible to see. But they won't listen. Some will, the other ones won't and they're the ones that keep dropping."

The recent deaths have prompted police and the chief coroner to issue a public warning about synthetic cannabis.