National has gained support in the latest opinion poll and is close to 50 per cent while Labour has slumped to its lowest rating in two years.

The just-released Roy Morgan poll gives National 49.5 per cent, up 1.5 points in a month, while Labour has shed 3.5 points and has fallen to 23 per cent.

The Greens have gained three points to reach 14.5 per cent, giving a Labour/Greens alliance 37.5 per cent.

NZ First is down two points to eight per cent.

A Colmar Brunton poll released on Sunday gave Labour 28 per cent.

Labour released its private polling in September in reaction to a Colmar Brunton poll that put the party at 26 per cent, but it doesn't intend doing that this time.

NZ Newswire understands its private, pre-earthquake polling shows support slightly higher than 28 per cent, with Labour and the Greens almost neck and neck with National.

The latest Roy Morgan poll shows that if an election was held now, the government would be re-elected.

There's little change in the ratings of the minor parties.

The poll was conducted during November and covered the Kaikoura earthquake.