"The devil is in the detail," a Labour adviser said of Labour's proposal to impose a levy on workplaces which were not training New Zealand workers up.

An hour later, Grant Robertson was indeed in a form of hell as he tortuously tried to explain how a policy which looked, smelt and quacked like an attempt to penalise companies for hiring migrant workers was not that at all.

The proposal had looked fairly obvious. It was in a section of the Future of Work report that spoke of "skilled immigration being used to compensate for our failure to train our own local workforce for the jobs that are available."

The levy was to apply to companies in sectors with skills shortages which were therefore reliant on migrant labour.

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Earlier, Labour leader Andrew Little had rattled off the number of work visas given out to semi-skilled workers and said it "didn't make sense." The examples he had used of businesses which might be affected had been construction, chefs and IT - all of which are heavily dependent on migrants.

Robertson also emphasised that the aim of it was to train up the New Zealand workforce, the young ones without jobs, education or training opportunities.

So it was a fair assumption the levy was aimed at promoting local workers over migrant workers.

But no.

Asked if it was a crack down on migrant workers, Robertson said companies would not have to pay the levy if the workers they were training were migrants either.

The proposal which began life as a way of getting New Zealanders into skilled jobs instead had transmogrified.

It had turned into a proposal to get anyone into skilled jobs, whether they came from France, China, the Lower East Side of the Planet Melmac or Eketahuna.

Robertson even re-cast it as a business rights issue, rather than a worker rights issue, saying some businesses were "pissed off" because others were taking all their trained staff without training any of their own.

By this point the detail (what there was of it) was getting very devilish indeed.

There was no real need for Robertson to baulk at the migrant worker question - there is, after all, nothing wrong with a political party putting the rights and opportunities of New Zealanders over others.

But Labour had felt the sting of accusations of xenophobia over its Chinese surnames business.

Robertson had sensed swiftly where things were heading and hit reverse.

Labour must have known the proposal would get some attention. It had not spoken simply of training young workers, but had thrown the concepts of migrant labour versus 'New Zealand workers' into the mix.

It was the one measure of 60 recommendations that the party chose for Little to release on television on Saturday morning. It had crapped in its own nest.

The trouble is if you tap on the dog whistle, it doesn't take long for the entire orchestra to kick in behind it.