New Zealand Prime Minister Jacinda Ardern sacked a senior cabinet minister Wednesday over an affair with an ex-staffer, amid heightened scrutiny on lawmakers' behaviour ahead of a general election in September.

Ardern dismissed Immigration Minister Iain Lees-Galloway after discovering he had an affair with a former colleague who was working at a government organisation that reported to him.

The centre-left leader said Lees-Galloway's role as minister for workplace relations, which regulates employment conduct, made his position untenable.

"The minister has shown a lack of judgement over a period of 12 months -- in undertaking this relationship he has opened himself up to accusations of improperly using his office," she told reporters.

Ardern said she was not passing moral judgement on Lees-Galloway but added: "He has not modelled the behaviour I expect as a minister that is in charge of setting a standard and culture in workplaces".

Lees-Galloway, who is married with three children, issued a brief statement admitting he acted "completely inappropriately" and apologising for letting his family down.

Details of his affair were passed on to Ardern's office on Tuesday by opposition leader Judith Collins, who had urged the public to contact her with allegations about inappropriate parliamentary behaviour.

Collins' appeal came after one of her colleagues in the centre-right National Party, Andrew Falloon, quit this week over a series of unwanted "sexting" messages sent to young women.

Collins became opposition leader last week promising to "take the fight" to Ardern in the September 19 election.

Ardern's Labour Party maintains a healthy lead over National in opinion polls and Lees-Galloway's sacking is unlikely to dent her chances of winning a second term.

However, the ability of the new opposition leader, nicknamed "Crusher Collins" to take a prized ministerial scalp indicates the contest will be keenly fought.