Francis Monopoli, owner of Nelson's Waka restaurant, said that all cakes will be clearly labelled in future to prevent any further mix-ups in the kitchen.

A group of people ended up in hospital after being served a birthday cake that was allegedly laced with cannabis at a Nelson restaurant.

Police have investigated the complaint and found insufficient evidence to lay charges, saying "no cake was seized at the time".

The cake was served at a birthday dinner celebration on July 3 at The Waka restaurant in Collingwood St.

The restaurant provided a complimentary birthday cake with the booking.

The Waka owner Francis Monopoli, who was not working on the night, said the restaurant chef baked a cake, but whether or not that cake was served to diners was uncertain.

Diner Nicola Nish said the cake the group was served "tasted weird".

"It just had a really weird texture and a weird flavour to it."

Nish said a short time later, several people who had eaten the cake started feeling strange.

"We were all a wee bit different to tell you the truth. I was quite jittery," she said.

She said her husband was also "jittery" and his heart was beating faster than normal, so she called an ambulance.

"That was quite scary actually, not knowing what was going on."

The Nelson Marlborough District Health Board confirmed that a group of people was treated at Nelson Hospital on July 3 after eating cake that was allegedly laced with a type of cannabis.

A spokesperson said about six people were treated but could not comment on symptoms, treatment, or the results of medical tests for privacy reasons.

The health board and Ministry for Primary Industries (MPI) investigated the "suspected food borne illness".

"MPI began making initial enquiries as per standard protocol," compliance operations manager Gary Orr said.

"However, once the presence of cannabis (an illegal substance) was confirmed, MPI concluded their investigation and referred this matter to the police."

Sergeant Sally McBride said the police investigation has been completed.

"While police believe it is highly likely that these people were served a cake that was laced with cannabis, there is insufficient evidence to prove this," she said in an emailed statement from the police national media team.

"The people involved underwent tests, which confirmed the presence of cannabis. Unfortunately, at the time of reporting no cake was seized for examination."

Monopoli said he was concerned about the allegations and although they were unsubstantiated, he did not question that the diners had consumed cannabis somehow.

"I don't know what happened," he said. "At the moment it's speculation."

He said no action had been taken against staff working on the night.

Nish said the group was fortunate that the incident wasn't more serious. "It was a great night until that happened."

She said it was a "wee bit disappointing" that police couldn't find evidence to lay charges "not that I want anyone to lose their jobs or anything".

"There was plenty of cake when we left the restaurant. What happened to it, I have no idea."

She said the "biggest lesson" to come from this was that restaurants needed to ensure that they labelled what was in their chillers.

Monopoli said he intended to do just that with every cake that came out of his kitchen in future.

"We had a staff meeting last week and I said we're going to take a sample of every cake now and freeze it, so if something does happen along these lines ever again it will be dated, named.

"That's just a preventative thing," he said. "We're pretty serious about food safety here."