New Zealand police are investigating a threat to contaminate baby formula.

They say it could be linked to suspected environmental activists opposed to the use of pest control poison. 

Letters and packages of baby formula laced with the poison were sent to the national farmers' group and dairy giant Fonterra in November.

The Primary Industries Ministry is seeking to reassure consumers. 

Dairy products make up a quarter of New Zealand's export earnings and more than seven per cent of its GDP.