Councillors were to debate the GMO ban last month but changes to the Resource Management Act meant a more detailed cost-benefit analysis had to be carried out first.
That analysis questioned claims that GM crops increased productivity, giving evidence that similar or better gains could be achieved through breeding. It also highlighted the potentially huge costs and loss of export markets, jobs and economic growth that would follow GMO contamination of the food chain.
"It is unclear whether economic growth and employment would increase as a result of the plan change. However, it would ensure these were not reduced by GM contamination incidents," the authors said.
The report added that Northland producers could benefit from marketing opportunities provided by a GMO ban. An informal GE-Free Zone would be cheaper than a formal plan change, but it would not be legally enforceable.
The costs of the plan change and any legal challenges will be shared with Whangarei. Auckland has similar provisions in its Unitary Plan.