Immigration New Zealand's improved border processes and ability to track overseas visitors have significantly reduced overstayers, an immigration expert says.
Overstayer estimates as of end of last year by the agency showed overstayer numbers had nearly halved from around 20,000 a decade ago to 10,894.
Massey University sociologist Paul Spoonley said a big shift had been the drop in overstayers from Samoa and Tonga.
"There are more than 6000 fewer overstayers from these two countries since 2000, and this accounts for 63 per cent of the drop," Professor Spoonley said.
"There are several reasons; one is the scrutiny that has increased in recent years, and the second is that the overstayer numbers have dropped since the end of the Global Financial Crisis suggesting that the growth in labour demand has accounted for some of those who have found jobs under the transition visa arrangements rather than simply stay on illegally."
Spoonley said programmes such as the Recognised Employer Scheme had provided a legal employment option, especially for Tongans - the top nationality of overstayers.
"Migration from Samoa and Tonga is very modest, about 3000 per year, although the visitor numbers are significant at 45,000 per year," Spoonley said.
Nearly five and a half million visitors came to New Zealand in the last year, with 2.7 million being non New Zealanders.
A key reason for the significant drop is overstayers was the agency's improved capabilities to track people and border processes, he said.
Most of the overstayers were previously on a visitor visa (6735), followed by work visa (1981) and student visa (1523).
Immigration said it had also focused on improved security at the border.
In the year to June 30, the agency said 1207 people were denied entry to New Zealand at the border.
This number was also down from the 1371 during the previous financial year.
Malaysians were the top nationality of people refused entry, followed by Australians and Brazilians.
Senta Jehle, the agency's border national manager said reasons included not being considered to be genuine tourists and likely to remain in the country unlawfully.
Another reason was failure to meet character requirements for the grant of visa, which usually related to not declaring serious criminal convictions, deportation or exclusion from another country.
Immigration Fay Holdom said fewer removal and deportation also meant significant savings for the taxpayer.
According to Holdom, deportation costs had "fallen dramatically" from $3 million in 2006 to $1.7 million last year.
Last year, 747 overstayers were deported, and the agency also negotiated voluntary departures of a further 1437 - where people paid their own costs, but could apply to return to New Zealand without a travel ban.
"The huge drop in the estimated number of overstayers can be attributed to...decisive action to deal with overstayers," he said.
"We automatically know when someone has not departed before their visa has expired, we contact overstayers through texting, email and letter and when they fail to depart, compliance staff undertake enquiries to locate and deport."
Holdom said better risk profiling and an increased focus on encouraging overstayers to settle their affairs, pay their own costs for departure and leave the country voluntarily had also contributed to the significant decrease in numbers.
Overstayers in NZ
10,894 estimated in NZ
747 deported last year
1437 agreed to leave