Overseer under fire

New Zealand needs to upgrade the Overseer nutrient management tool if it wants to use it as a way of regulating farm pollution.

A new report says the public does not understand how Overseer works, and recommends it should become an open-source model.

The Parliamentary Commissioner for the Environment, Simon Upton, queried whether councils could be confident in making regulatory decisions based on Overseer’s estimates of nutrient loss.

The tool was initially used by farmers to make more efficient use of nutrients but increasingly has been seen as a way of controlling pollution.

Of the 16 regional councils, eight use it to regulate pollution. However there has long been controversy over how accurately it measures nutrients.

Upton acknowledged the limitations of Overseer. ‘‘It is time to open up Overseer. If the Government wants to see the model being used as a regulatory tool then a large measure of transparency is needed.’’

He said farmers and regional councils needed to be confident that Overseer’s outputs were reliable.

A not-for-profit company Overseer Ltd runs the model, which is jointly owned by the Ministry for Primary Industries, the Fertiliser Association and Agresearch. Over its 18-year life, more than $12m has been spent on developing it.

Hamilton-based agricultural economist Phil Journeaux, who wrote a report in 2016 saying it benefited the rural economy to the tune of $271m a year, said Overseer was not without its critics but it was the best tool that existed. ‘‘At the moment it’s a case of ‘there is no alternative’,’’ he said.