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Mr Key said the data from the Statistics New Zealand household labour force survey out today was at odds with the quarterly employment survey.
According to the household labour force survey the unemployment rate rose to half a percentage point to 7.3 per cent in the September quarter, the highest level since June 1999.
Economists surveyed by Reuters were picking a 0.1 percentage point fall to 6.7 per cent.
Full-time employment shrunk 0.8 per cent to 1.7 million, while part-timers rose 1.4 per cent to 519,000.
"We're pretty surprised by the data. It's at odds with quite a lot of other data that we see," Mr Key said.
Today's figures come after the quarterly employment survey showed total filled jobs rose 0.3 per cent to a seasonally adjusted 1.715 million, bolstered by a pick-up in part-time workers and a decline in full-time equivalents to 1.35 million.
Mr Key also said data showing an unemployment rise in Auckland was at odds with "anecdotal evidence".
He said the Government were committed to their economic plan and there would be no change of tack after the survey was released.
"We're on the right track, in our view - we are reforming the economy as, is absolutely necessary.
"Things will bounce around, you will have international conditions to contend with - but from New Zealand's point of view we've grown 1.6 per cent in the first quarter, we've grown 57,000 jobs in the last 12-18 months.
"It is at odds with what most of the economists thought would happen; it's slightly unusual when we see what's happening in terms of those on the unemployment benefit - but that's the way statistical things happen.
"These are challenging international conditions -we see that with Germany likely to be growing by only about 0.8 per cent and the rest of the world slowing down.
Mr Key said he would wait for future data to assess if there was really a problem.
- By Kate Shuttleworth of APNZ and Paul McBeth of BusinessDesk