After studying the costs and benefits of their proposals, I concluded that they appeared costless because there was no commercially viable GMO that would be blocked by the councils' proposed course of action. Lots of people wanted the GMO-free branding (including some very large exporters), so the net effect seemed to be "some gain, no pain".
Not finding evidence doesn't prove it isn't out there however. Fortunately, local government processes are highly contestable, so my evidence was open to challenge, including by GMO fans who'd naturally be best placed to identify any gems I'd missed.
There was indeed plenty of challenge, including some very unprofessional and unfounded personal attacks on my professional integrity by people employed by Scion and Federated Farmers (the Feds guy at least had the decency to apologise later). Emotions can get the better of people at times.
On the crucial evidential question though, there was complete silence. No-one ever pointed to an actual GMO, ready for commercialisation now, for which there was grower demand. Nor could those opposing these local governments even identify a GMO that is close to being ready.
This matters, because the plans only last for 10 years and there are long development horizons for new GMOs. If there was an outdoor GMO ready to go now, that local growers wanted, then there would obviously be a cost in prohibiting its use. Even if there were reasonable expectations that this would occur within a few years, it might be better to not prohibit them.
The cupboard was bare however, so the councils were not imposing any costs on their populations by proceeding as they have.
But science is constantly advancing, so maybe new evidence has come to light in recent months? If so, the president of Federated Farmers doesn't know of it, judging from his studious avoidance of this question on Twitter today.
@FedsPres Can you tell us of an outdoor cultivated GMO, on the market, that NZ farmers want to grow now but can't?— John Small (@smalltorquer) September 5, 2016
So at this point, the Feds' emperor looks stark naked.