The number of people crossing New Zealand's border has climbed since the two-way quarantine-free Trans-Tasman travel bubble opened with Australia on April 19, the statistics department Stats NZ said on Monday.

According to Stats NZ, there were 85,900 border crossings in April, made up of 47,000 arrivals and 38,900 departures, reports Xinhua news agency.

Total border crossings were well up compared with the earlier months of 2021, which averaged 26,300 border crossings a month, Stats NZ said.

"Despite this increase, arrivals and departures are significantly lower than levels before Covid-19, when border crossings were as high as 1.2 million in April," population indicators manager Tehseen Islam said in a statement.

Border crossings include all arrivals and departures, either for short-term trips or longer term, and by people living overseas or in New Zealand, according to Stats NZ.

There were over 70,000 border crossings to and from Australia in April 2021, made up of 39,900 arrivals and 30,200 departures, it said.

"Trans-Tasman travel made up about 80 per cent of all international travel in April 2021," Islam said, adding New Zealand citizens represented about 60 per cent of arrivals from Australia and 67 per cent of departures to Australia.

"With the introduction of quarantine-free travel between New Zealand and Australia, the main reason for people travelling was to visit family and friends."

Provisional border crossing data up to June 6 showed a small increase in travel with the Cook Islands each week since two-way quarantine-free travel opened on May 17.

However, this is well below levels in the same period in 2019, he said.