Support for New Zealand's Prime Minister Jacinda Ardern rose during July, a poll showed, keeping her Labour party on track to win a general election in September.

The 1 NEWS Colmar Brunton Poll showed Labour's popularity at 53%, a rise of three percentage points from late June.

If repeated in the election, that would allow the party to govern on its own rather than in coalition.

The opposition National Party dropped six percentage points to 32%, after a series of scandals and a change of leader.

Support for incoming Nationals leader Judith Collins rose to 20% on a preferred prime minister basis, far higher than previous party leaders, but still well behind Ms Ardern, whose popularity was steady at 54%.

A Nationals lawmaker resigned this month after being accused of sending a sexually explicit image to a young woman, while another resigned after admitting to leaking the names of coronavirus patients to media.

A News hub-Reid Research Poll released on Sunday showed Labour's popularity rose to 60.9%, its highest in the poll's history.