At least 20 people have now reported symptoms of food poisoning to health authorities after eating spoiled fish from food delivery service Hello Fresh.

An Auckland Regional Public Health Service (ARPHS) spokesperson said that in the last 24 hours it has received around 20 calls from people who ate the food provided by the meal kit company.

When fish isn't chilled properly, it can produce histamine that can be toxic to humans leading to scombroid poisoning, according to the New Zealand Food Safety website.

Meanwhile, further south, the public health unit for the greater Wellington region says it has also received a number of phone calls from "concerned members of the public on this matter."

Hello Fresh customers were warned yesterday to ditch the fish dish in their packs after the home delivery service's fish supplier advised of elevated histamine levels in the trevally.

Three people were treated and discharged from hospital yesterday after eating the trevally meal.

ARPHS medical officer of health Dr Jay Harrower says symptoms of histamine or scombroid poisoning are often a combination of an allergic reaction and gastroenteritis.

Symptoms include tingling and burning around the mouth, facial flushing, and diarrhoea, skin rash, vomiting, dizziness, possibly respiratory distress and can last up to 48 hours, he says.

It doesn't usually result in any long term effects, he says.

"The illness typically begins within 30 minutes to an hour or two of consuming the fish," Harrower says.

"There are some fish varieties which are more likely to produce histamines at levels that are toxic, often when the fish is not kept chilled."

One of those varieties is trevally.

New Zealand Food Safety is investigating how the spoiled fish ended up being delivered to homes across the country since Saturday.

New Zealand Food Safety's food compliance national manager Melinda Sando said it was too early to consider prosecuting the company for the issue.

"Our focus at present is to establish the cause of the issue so that it can be addressed and the company can prevent its recurrence," she says.

"New Zealand Food Safety has a range of powers under the Food Act to require businesses to take appropriate steps to ensure the safety of consumers."

Failure to adhere to directions can lead to a range of penalties, she says.

"Ultimately any penalties would be decided by the Courts through sentencing following a New Zealand Food Safety prosecution."

In a message to customers who ordered the meal kit, Hello Fresh apologised for the issue.

"We sincerely apologise for any inconvenience caused and will be providing a refund of the cost of the ingredient. We want to assure you the safety and quality of our ingredients is our highest priority."

The trevally was an ingredient in a coconut, lime, and shallot dish.