The left's brief July day in the sun clouded over in August. The August four-poll average shows National comfortably back ahead of the combined Labour-Greens support.

Photo: RNZ

The July average gave Labour-Greens a 0.9% lead.

It's good news for National that it has managed to nip ahead again despite growing pessimism about the economy; TV1's Colmar Brunton poll published on 9 September had pessimists at 45% and optimists at 33%.

The lead also comes despite issues such as the Saudi sheep deal and fights at Mt Eden prison, though John Key's misreading of the public - and his support parties' - mood over Syrian refugees came too late for the polls.

After a slight downtrend through this year to a low of 44.5% in July, National's average has picked up to 47.6%, above its 2014 election score. A year into its third term, that is an unusually strong reading.

Labour, by contrast is stuck around 30%. It briefly nudged over 32% in July when it made a big deal of Chinese buying Auckland houses but was back to 30.3% in August - around where it was in February.

Andrew Little has work to do. The upward trajectory in his first few months as leader appears to have plateaued. And he continues to trail John Key as preferred prime minister by wide margins. One glimmer of hope is that it often takes time for worsening economic sentiment to translate into a vote shift.

The Greens have been stuck between a 10% and 13% range all year, despite getting themselves a presentable new co-leader in James Shaw. In August they averaged 11.7%.

Graphic: RNZ

That left National 5.7% ahead of Labour and the Greens combined.

Graphic: RNZ

New Zealand First has not been able to build on Winston Peters' Northland by-election win in March. Its August reading was 7.6%, almost exactly where it was in July.

Among the minor parties, only the Maori party cleared 1% and then only just.

Explainer: The poll of polls is an arithmetical average of the four most recent major polls since the election from among: TV1 Colmar Brunton, TV3 Reid Research, Fairfax Media-Ipsos, NZ Herald DigiPoll, Roy Morgan New Zealand and UMR Research, which is not published. The four polls in the most recent average were, in order of interviewing, Morgan, UMR, DigiPoll and Colmar Brunton, all in August. The first point on the charts is the actual election result and the polls averaged in the next three points straddled the election. The first point for which all polls were taken after the election is in mid-November.