The Prime Minister is set to get a briefing on the mass outbreak of campylobacter in Havelock North as the Government also looks to start its own inquiry.
Hundreds have fallen ill since last Wednesday as the town battles an outbreak of gastro - the council confirmed it was likely to be an E. coli contamination on Friday.
A number of patients have been admitted to hospital while on Sunday alone, 183 people turned up at their GPs with symptoms of campylobacter infection.
John Key said the Government has already offered its support to Hastings Mayor Lawrence Yule, and acting Health Minister Sam Lotu-Iiga has also been getting regular updates on the situation.
"The focus at the moment is on getting people well and ensuring others don't get sick. In time, however, we need to establish how this has happened and how it can be prevented from happening again," he said at his post-Cabinet news conference.
He will get a briefing on the situation on Tuesday, along with Mr Lotu-Iiga and Associate Health Minister Peter Dunne.
He said the council will be conducting an independent review into what happened and it was "likely" the Ministry of Health would also start one. However, it wasn't known if it would be independent of the council or as part of it.
"I don't think it is acceptable. There are a large number of people affected; we certainly know there are dozens of identified cases of campylobacter. We need to understand how this material has got into the water supply."
Mr Key says given the number of people who have become ill, and the dilution of the bacteria in the water which would be expected, it is "clear... that quite a lot of material has got into the water source".
A number of schools have closed because of the illness.