New Zealand had 258,200 overseas visitor arrivals in September 2018, up 5,400 from the same month last year, the country's statistics department Stats NZ said on Wednesday.

Australia remains New Zealand's single biggest source of visitors, but an increasing number of visitors came from Asia in September, which is typically a lower season for tourists, Stats NZ said.

Although nearly half of all visitors in September 2018 were from Australia, the number was down 3,500 from last September, it said.

In contrast, visitor arrivals from Asia were up 5,100 compared with last September, continuing an ongoing trend, with the number of visitors from the Chinese mainland up 1,700 and Malaysia up 1,600, statistics showed.

Visitors arriving for the main purpose of a holiday reached two million in the year ended September 2018. This was up 83,300 from the year ended September 2017, the first time a September year has reached this level, the department said.

Total visitor arrivals for the September 2018 year were 3.81 million, up 130,900 from the September 2017 year, statistics showed.