This was made up of a net gain of 7200 New Zealand citizens, and a net loss of 4700 non-New Zealand citizens.
"The net gain of New Zealand citizens continues a reversal of the long-standing historical pattern where more New Zealand citizens depart than arrive," population indicators manager Tehseen Islam said.
"Overall, however, migration flows since April 2020 remain well below those seen in previous years.
"Net migration has been high in recent years, and the six months from April to September would typically see a net gain of at least 20,000 migrants."
Westpac chief economist Dominick Stephens said he found it surprising that a narrative around a historic wave of Kiwi arrivals was taking hold.
The much bigger part of the story this year was that there were far fewer numbers of people crossing our borders - in both directions.
"There's no real evidence of a wave," he said. "In fact, it's stronger than that. There's evidence the opposite of a wave is occurring.
"There is always a steady flow of Kiwis arriving in the country," he said,
"Over the past six months, the number of Kiwis arriving long-term has actually averaged half of the normal number. So there is an unusually low number of Kiwis arriving at present."
Meanwhile, Stats NZ has been collecting new data about the occupation of the returning Kiwis and where they plan to settle.
"Travellers arriving into the country have identified with a broad and diverse range of occupations, however, we don't know whether these people will be looking to join the New Zealand workforce," Islam said.
From April 2020 to September 2020 the stated occupation of overseas resident arrivals has included teachers (1000), engineering professionals (900), miscellaneous labourers (for example, ship crew and general labourers (800), midwifery and nursing professionals (600), and miscellaneous specialist managers (600).
About half (26,900) of all arrivals from April 2020 to September 2020 had an occupation allocated.
Occupations weren't allocated to students, retirees, children and people who didn't provide a response, or whose response couldn't be identified. Occupation information is not gathered for departures.
It remained to be seen how many overseas resident arrivals will stay longer than 12 months and be classed as migrants, but current provisional estimates are about 24 per cent.
Just over 2 in 5 (about 13,000) overseas resident arrivals from April 2020 to September 2020 provided a street address in the Auckland region.
By comparison, just over 1 in 3 (about 1.7 million) of New Zealand's population lives in Auckland.
"Most overseas residents are providing street addresses in North Island regions, particularly Auckland, however they may end up staying elsewhere during their time in New Zealand," Islam said.
Overseas resident arrivals have also provided addresses in other regions, including Canterbury (3000), Wellington (3000), Waikato (2500), and Bay of Plenty (2100).
A recent survey of 15,000 expats by KEA (Kiwi Expats Association) found that 49 per cent were planning to come home.
About half of them said they would be returning in the next two years.
There are about 1 million New Zealanders living overseas according to Stats NZ, so the survey provided a larger sample by the standards of most polls.
It prompted some to assume that as many as half a million Kiwis may return home.
However, Westpac's Stephens was sceptical.
He noted that logistics of a large wave of returnees, while borders were closed, meant it was unlikely to be a factor in economic forecasts in the foreseeable future.
For now, the much more important dynamic was a sudden drop in population growth, he said.
The survey drew on the KEA membership and its wider network of expat Kiwis.
