Stressing that National was busy removing millstones which Labour had hung on the economy when in office, Gerry Brownlee astonished Opposition MPs by then placing a potential millstone around his own neck yesterday.

It was so unnecessary. Given he had plenty of notice that Labour had lined him up for a parliamentary question about income parity with Australia, the Minister for Economic Development should have come down to the House armed and primed with every last statistic on the matter.

And not just for his own political good. National's 2008 election campaign placed huge stock on putting New Zealanders' living standards on a par with Australians' by 2025.

National might now describe that target as "aspirational". But it knows voters expect some measurable progress.

Brownlee's mauling last week after the Cabinet's u-turn on his plan to allow mining in national parks and other parts of the conservation estate has only further underlined the question of how National plans to close the transtasman income gap.

Asked by Labour's David Parker if he had any targets or milestones to achieve income parity by 2025, Brownlee replied "yes", but did not expand on what those might be. That was politically sensible, given missing targets could prove embarrassing and Brownlee has soaked up enough of that in the past week or so.

Parker pressed on. He then asked what the current average weekly gap in income was between a New Zealand and an Australian full-time worker.

Brownlee replied that it was "certainly a lot less than it was when Labour was in office". But he then added that he was not familiar with exactly what the gap was. Neither - when pressed further - could he give an approximation.

He was "more than happy" to provide an answer if Parker asked a written question. It would take "quite a bit of time" to work out exactly how much the gap will have closed when National's tax cuts kick in from October.

Brownlee won't get that time. Or the luxury of weeks to answer a written question.

Hardly believing his luck and predicting Brownlee had got it badly wrong, Parker told the House he would be submitting an oral question for the minister to answer today.

Ministers are given more than three hours' notice of the questions they are required to answer, though not the further supplementary questions Opposition MPs put to them.

Like any other minister, Brownlee can call on his departmental officials to come up with the answer. Despite his protests, that should be relatively simple to work out.

Senior Labour MPs were clearly amazed that Brownlee did not have that information at his fingertips yesterday, seeming to prefer to wing it, but with less confidence he was right the longer the questioning went on.

Parker asked if Brownlee stood by his first answer that the income gap has decreased since National took office.

Brownlee's reply was not so categorical this time. "I believe that that is the case."

Brownlee and Parker cannot both be right. But there is no doubt which one will be in the parliamentary dog-box if he is wrong.