The latest New Zealand Roy Morgan political poll shows the National Party's lead in polls has been cut to its smallest margin since September, as New Zealand First leader Winston Peters takes a strong stand on the flag referendum and support for his party is right up.

The poll shows that during March support for National fell 2.5 per cent to 46 per cent, only four per cent ahead of a potential Labour-Greens alliance on 42 per cent, up 0.5 per cent.

If an election were held now, New Zealand First, which is up three percent in the poll, would hold the balance of power and be in a position to determine who would form the next government.

Support for National's partners is little changed.

The Maori Party sits at one per cent, unchanged, while ACT is one per cent, up 0.5 per cent, and United Future is unchanged on zero per cent.

Labour's support is up one per cent to 28, the Greens are down 0.5 per cent at 14, and New Zealand First has jumped three per cent to nine per cent, its highest support in more than a decade.

NZ First was last in Government under Prime Minister Helen Clark between 2005-08.