Formula threat: 2000 persons of interest

Photo / John Borren

Almost 2000 people are being viewed as persons of interest in the investigation into the threat to contaminate infant formula with 1080 poison.

Authorities were warned they had until today - March 31 - to stop using 1080 as a form of pest control or the protester would release the controversial poison into baby milk.

However, the list of potential suspects has soared to almost 2000, Police Deputy Commissioner Mike Clement said, as he admitted police may never get to the bottom of who's responsible for the threat.

"In terms of the investigation we're working through a significant number of people who are of interest to the investigation, and they number in the hundreds, actually nearly 2000 of them," Mr Clement told Newstalk ZB's Mike Hosking breakfast show.

Not all of them were suspects, he said, but "there will be some of those among that 2000 who are of more interest to us in terms of being a suspect".

But he admitted that "it's a possibility" police may never trace who was responsible for the threat.

"I'm very hopeful, we're very committed to it," he said.

"We've treated it seriously from the outset. We're well used to working on investigations that take an awful long time - that's the nature of the challenge that we have in front of us, and this is another one of those.

"It's certainly not unheard of, in my own experience, to work on investigations for months, if not years, before we get a successful conclusion."

Police were "really encouraged" by the number of phone calls from members of the public with information, he said.

"There have been over 600 of them. That's what we asked for .. and we just want to encourage people to keep doing that, so the more people that ring, the better," Mr Clement said.

"It does create a corresponding number of people that we've got to reach out to, but that's what we've been setting ourselves up for from the get-go."

Four months into the investigation, there was nothing to suggest the threat-level had increased, despite hitting the deadline to stop 1080 use today.

"We've always assessed it as being a low risk, and I'm not seeing anything currently that takes us away from that," Mr Clement said. "But we've, by necessity, got to treat it as a serious threat. We have and we'll continue to do so."

He "sincerely hope[d]" there would be no contamination following today's deadline, he said.

"But the reality is we can't afford to be complacent about it, it's a threat of significant proportions if it was to be genuine, and we've treated it as such from the outset."