WELLINGTON, March 11-- The blackmailer behind the New Zealand infant formula poison threat is probably a loner with personal issues, a leading psychologist said Wednesday as the government came under fire for describing the threat as "eco- terrorism."

The threat to poison infant and other formulas with 1080 poison appeared to be from a more personal than political motivation, said Massey University forensic psychologist Dr Mei Williams.

"They may have experienced a beloved animal die in severe pain from 1080 poisoning, hence the anger towards the authorities and the desire for revenge. For this reason, I don't see it as a group, but rather an individualized act, and in some ways has a similar feel of extremist groups believing the end justifies the means," Williams said in a statement.

"The person, or people, sees their relationship with animals as more important than human relationships -- hence the threat of harm to babies through poisoning of formula."

The person might feel a sense of satisfaction and empowerment in the wake of the media storm at having created "devastation and fear."

She thought the term "eco-terrorist" was inappropriate as " there is something honourable in that label. There's nothing honourable in what this person has done."

The main opposition Labour Party said Wednesday that the use of the term "eco-terrorism" by Prime Minister John Key and Minister for Primary Industries Nathan Guy was unfortunate.

"It has already played into the hands of our international trade competitors after the phrase was repeated in overseas publications," Labour primary industries and food safety spokesperson Damien O'Connor said in a statement.

"John Key is overplaying what is effectively the work of an independent idiot and nutter."

The opposition New Zealand First party accused the government of harming New Zealand's international reputation by deliberately creating a political crisis over a crank letter threatening to contaminate baby formula.

New Zealand First leader Winston Peters claimed major food and drink manufacturers said they received crank letters all the time and they had set rules to following such cases.

"They are saying John Key and Nathan Guy are going about this issue the wrong way and are in fact feeding the appetite for these idiots and encouraging copycat behaviour," Peters said in a statement.

Police and food safety officials have asked for the public's help in finding the person or people behind the threat, which was issued in November last year and made public on Tuesday.

Police said the letters sent to the Federated Farmers industry group and the Fonterra dairy cooperative were accompanied by small packages of milk powder that subsequently tested positive for the presence of a concentrated form of the poison 1080.

The letters threatened to contaminate infant and other formula with 1080 unless New Zealand stopped using 1080 for pest control by the end of March 2015.

Sodium monofluoroacetate, known as 1080, is a poison used to protect New Zealand's native flora and fauna against introduced pests such as possums and ferrets.

Its use has been controversial over the years with opponents saying it poisons non-target animals and contaminates the environment.