Of those surveyed, 72.7 per cent agreed he could not be blamed because the increase was due to economic factors beyond his control, while 24.6 per cent said the numbers dented his credibility.
National campaign spokesman Steven Joyce said he was not surprised by the results. "We've always said that it's a long job in terms of closing the income gap with Australia, and it's not going to happen in five minutes."
The recession, the global financial crisis and the Canterbury earthquakes had contributed to people leaving, he said, and voters understood those factors were beyond the Government's control.
However, Labour campaign spokesman Grant Robertson said Mr Key had avoided talking about the issue so the facts were not entirely clear to the public.
"We all acknowledge that there have been major challenges to the country, but when you look at the detail of the statistics you'll see that there has been a steady increase under his watch. While the poll results are what they are, we believe this is in part because of a lack of economic plan."
Voters were divided over their concern about people moving to Australia, with 50.1 per cent saying they were not worried by the number leaving, 41.6 per cent saying they were concerned, and 7.9 per cent saying they were extremely concerned.
Nearly three-quarters, 74.2 per cent, said they had not seriously considered shifting to Australia, 9.3 per cent said they were considering moving, and 15.3 per cent said they wanted to move but circumstances were stopping them.