"They are often cheap in terms of labour or wage costs and they are often meeting labour needs that cannot be met by local workers," he added.
AUT University business school researcher Danae Anderson warned that this emerging trend "should be of concern" to New Zealand agencies.
"In addition though the concern should be the conditions these overstayers are living in, given that they have no welfare support and the only jobs available to them are in the secondary labour market with its exploitative and precarious conditions," she said.
"As overstayers they will typically be deported so become 'invisible' so as not to draw attention to themselves."
The most recent estimate of overstayers, carried out in September last year, put the total at 13,151.
Skilled job vacancies grew 0.6 per cent last month, and were up by 10.1 per cent for the year according to the Ministry of Business, Innovation and Employment.
Licensed immigration adviser Tika Ram, who is representing seven clients unlawfully here after they could not renew their work visas.
"Some have worked in the country for more than seven years...many of them have children who are not attending school due to their parents' immigration status," he said.
In the 2009/10 year, nearly half of the 166,857 work visas granted were renewals, or to workers who had previously held work visas.
Renewal approvals fell from 49.5 per cent in 2009/10 to about 43 per cent in the 2012/13 and 2013/14 years.
Between July 1 to September 30, just 38.5 per cent of work visas approved were to previous work visa holders.
Mr Ram said it was "unjust" for Immigration to be issuing new work visas instead of renewing previous visa holders who had jobs and employer support.
Vinesh Pratap, 39, director of Vinnies Insulation and Interior Lining Ltd, said he had been in a "constant battle" with immigration to get work visas renewed for staff.
His company employs 20 workers - five were foreign nationals who required work visas.
One, who had been working at the building company for five years, had his application declined while another two applications were still undecided.
Immigration denied there had been any change to its policy on approving work visas.
"Temporary work visas are issued for a limited period as labour market conditions can often change and employers need to prove that there are no New Zealanders available to do a particular job every time an individual work visa expires," an agency spokesman said.
"Immigration officers need to be satisfied that there are no suitable New Zealanders available to take up a job which has been offered to a temporary migrant."
An overstayer
A 29-year-old Punjab native who came to New Zealand on a work visa as an IT specialist became an overstayer after he failed to get a further work visa.
The overstayer, who spoke to the Herald on the condition of anonymity, spent $12,300 to an agent to get his initial one-year work visa.
"The agent told me after one year, I can get a renewal and then eventually become a permanent resident," said the man, who came in January 2012.
"I was shocked when my application to renew my work visa was declined even though I had a letter of support from my employer."
He came to New Zealand after friends who were here said they managed to get residency after working for several years.
His visa application was declined because Immigration New Zealand was not satisfied that his employer had done enough to ensure no suitable New Zealanders were available to take up the job.
After several unsuccessful appeals, he decided to remain unlawfully in the country because he wanted to "earn back" what he paid his agent and the amount he spent getting to New Zealand.
"I borrowed the money and would be a bankrupt if I went back home," he said.
"I thought that I would be working for several years at least, because if it was just for one year then it would not be worth my investment."
The overstayer now does odd jobs for cash payments, including as a kitchen hand in an Indian restaurant.
He would not say how much he has earned since becoming an overstayer or who his employers were.
