Support for John Key's National Party has fallen 2.5 per cent in the latest Roy Morgan political poll, suggesting a Labour-Greens-NZ First alliance would have the best chance of governing if an election was held today.

The poll of 894 voters conducted between April 30 and May 13 found support for National was 44.5 per cent, down 2.5 points from the previous poll.

Gary Morgan of Roy Morgan NZ said the continuing scandals involving Act leader John Banks appeared to be rubbing off on Mr Key's Government.

Support for Labour was 1.5 points higher at 30 per cent, the Greens were unchanged at 15 per cent and Winston Peter's NZ First was up half a point to 5.5 per cent. Together they had 50.5 per cent.

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Mr Banks' Act Party was down half a point to zero per cent while the Maori Party was also down half a point at one per cent.

Hone Harawira's Mana party was unchanged at 0.5 per cent. Other parties were at 2.5 per cent combined while 5 per cent of those polled didn't name a party.