The culprit for the current food poisonings is probably Vibrio parahaemolyticus, a bacterium found in marine, coastal and tidal waters, and most commonly causes gastroenteritis. It is not normally spread person to person, though poor personal hygiene, such as failure to wash hands after handling raw shellfish, could result in infections.
Symptoms usually occur within 24 hours of eating the infected food and include
- watery diarrhoea (occasionally bloody diarrhoea)
- abdominal cramps
- nausea
- vomiting
- fever
- headache.
Image from Pathogen Profile Dictionary
https://www.ppdictionary.com/bacteria/gnbac/parahemolyticus.htm
There is a feeling among some experts that Vibrio parahaemolyticus entered New Zealand in imported fish or shellfish. Cross contamination in the markets then allowed its spread.