Almost 85,000 people formally departed Australia in the final three months of 2017 — almost 9,000 more than in the same period in 2016.
But experts are unsure about exactly what is causing the increase.
The ABS measures a resident as someone who has been in Australia for 12 out of the past 16 months, so includes many foreign students and workers on temporary visas.
Anthony Grubb, director of demography at the ABS, said these "preliminary" figures show a "significant increase" in departures of foreign students.
Approximately one-third of departures in recent years have been Australian citizens moving overseas, while almost one-half are temporary visa holders like overseas students, foreign backpackers and 457 visa workers ending their stays.
Nick Parr, a demographer from Macquarie University, suspected the departure spike may be a reflection of growing numbers of students leaving at the end of the academic year, but it was too early to say definitively.
"In the past, temporary arrivals have been up and I would expect to see a bit of a lagged trend between the departures and arrivals on a long-term basis," Professor Parr said.
Department of Education figures show international student enrolments have increased from around 300,000 five years ago to 540,000 in February.
Migrants changing minds
Professor Parr speculated there may also be other reasons for the increase.
"It may be that with the tightening of the criteria of employer nominations that there is less of a flow-on from previous 457 visas to permanent visas, and other temporary residents getting permanent resident visas through that route," he said.
In April last year, the number of occupations with pathways to permanent residency was slashed as part of wide-ranging immigration reforms.
Leanne Stevens, national vice-president of the Migration Institute of Australia, said the changes were implemented with "limited clarity" and she raised the possibility that this may have had an impact.
"The people who are here from countries with equivalent living standards, where they have a choice of going back to another good job, maybe some of those people have formed a view that they don't want the uncertainty that Australia is presenting with them presently about their residency status," she said.
The Government is set to reduce the permanent migration intake in 2017-18 by approximately 20,000 people.
The high number of late 2017 departures meant Australia's "net overseas migration" figure for the full year decreased slightly compared to 2016.
Across the year, 240,000 more people came to live in Australia than those who departed.
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