National's lead in the polls is now the smallest it's been since September, while New Zealand First is banking support from Winston Peters' stand on the flag referendum.

The latest Roy Morgan poll shows support for National's partners is little changed, but the opposition parties are swapping places in the ranks.

Labour's up just one percent to 28 while Greens are down 0.5 percent to 14.

If an election was held now New Zealand First, which is up three percent, would hold the balance of power.

Meanwhile an academic thinks Winston Peters' good showing in the polls could be seen as a protest vote against Labour's Andrew Little.

Auckland University's Dr Joe Atkinson said it's not just because Winston Peters has taken a popular position on the flag debate.

He said John Key is losing popularity, but Andrew Little's suggestion of regulating bank interest rates and his lack of media know-how hasn't helped him.