There have been no prosecutions for environmental pollution in Whanganui in the past five years - and one expert says that is a bad sign.

There were no prosecutions through Whanganui courts in any of the last five financial years, according to Ministry of Justice figures, and Greg Carlyon of The Catalyst Group believes that indicates environmental compliance is not being enforced effectively.

Mr Carlyon led prosecutions for Horizons Regional Council up until 2011 and said there had been very few prosecutions in the region since then.

"I have no confidence that levels of compliance have improved - there's no data to support that."


Mr Carlyon said the regional council claimed it was working with people rather than against them but he didn't believe its methods were working.

"The evidence for that is that water quality is still declining ... we've got obvious non-compliance around the region in terms of discharges, stock in water and other things, and the message is not getting through."

He thought the regional council needed to consistently enforce its regulatory commitments and hold people accountable.

An Environment Ministry spokesman said enforcement of the Resource Management Act was the responsibility of local authorities, and prosecution was only one of the options and normally a last resort. Other options included abatement notices and enforcement orders, letters and verbal warnings.

Mr Carlyon believes councils are doing deals with parties once they begin enforcement proceedings.

"Those parties are effectively mitigating their way out of the legal action by making donations and contributions to council environmental work and other things," he said.

He didn't think there was enough transparency and said any negotiations should be done in the public domain.

He was also concerned there was too much political involvement in prosecutions. The majority of regional councils in New Zealand included councillors in determining whether enforcement action would be taken, contrary to Crown guidelines.

And he wanted more oversight of regulatory activity through the Parliamentary Commissioner or the Environmental Protection Authority.

In one recent high profile case, two companies were prosecuted after a container at Turoa skifield leaked more than 19,000 litres of diesel contaminating Raetihi's water supply.

Petroleum Services was fined $34,500 in the Taihape District Court and ordered to pay $20,000 costs to WorkSafe for its role in the 2013 incident. Ruapehu Alpine Lifts also pleaded guilty to four charges at an Environment Court hearing in Ohakune. Horizons was awarded $240,000 along with court and solicitor costs.

Nationally, the number of environmental pollution prosecutions has more than halved over the past five years. There were 555 prosecutions in the 2010-11 financial year and 234 last financial year.

University of Auckland Associate Professor Kenneth Palmer said he wasn't surprised the number of prosecutions had dropped.

Dr Palmer's teaching and research interests include environmental law, resource management law, and local government law.

He thought people were becoming better educated about their environmental responsibilities. Farmers had proper effluent treatment measures, and regional councils were making sure farms were complying with the rules and people weren't polluting the way they used to.