Key Points:

National's dominant lead in this month's Herald-DigiPoll is the result of overwhelming support among men, coupled with the backing of almost half of the women who were polled.

A gender breakdown of the poll reveals that National has 60.6 per cent support among males, miles ahead of Labour's 24.7.

While National has been stronger than Labour among males for some time, the gap has become so pronounced only in the past two DigiPoll surveys.

Among females - a group National has spoken of wanting to target - the John Key-led party is also ahead, with 49.6 per cent support compared with Labour's 37.5.

Another interesting aspect of the July poll is National surpassing Labour in the under $30,000 household income bracket - traditionally the domain of Labour.

National made inroads into Labour's support in this bracket in last month's poll but remained slightly behind. This month, it enjoys 41.1 per cent backing compared with Labour's 37.9.

The change should be read as indicative only, however, because once the poll is broken into five different income brackets the number of people in each becomes small.

National now leads Labour in every income bracket, with the party's support increasing more as income rises.

Geographically, the gap between National and Labour is the same in Auckland as throughout the country.

National leads in every age bracket and does best in the 30-39, 50-59 and 60-69 age groups.

Labour is closest to National in the 70+ age group, where the gap is 14.5 points. In this grouping New Zealand First registers 14.5 per cent support, while National has 47.8.

The other age group where Labour gets closer to National is 18-24, largely because National's support is limited to 45 per cent by the Greens polling a strong 12.5 and the Maori Party 10.

In the preferred Prime Minister ratings, Mr Key leads Helen Clark by 14.2 percentage points among men, but Labour's leader remains more popular among women.

Helen Clark has 50.2 per cent support as preferred Prime Minister among females compared with Mr Key's 41.6 per cent.