The agency conducts a labour market test for other employers wanting to recruit foreign labour, who must first prove that no suitably qualified New Zealand workers are available, or could be easily trained to do the job.
About 60 per cent, or 690, of the agency's visa officers are New Zealand-based.
Although it claims to regularly assess the market to make sure offshore staff are paid "market rates", they are still generally being paid significantly less than locally based staff.
The Herald understands in some places, like the Pacific Islands and Asia, officers are paid about a quarter of what their counterparts in New Zealand are getting.
"We know that [Immigration] is paying the market rate in the overseas country, but that is well below New Zealand rates," Ranson said.
"They could be employing New Zealanders to do this work."
Immigration assistant general manager Geoff Scott said the agency always had the ability to shift work overseas in response to changing processing demands.
Since it started to accept online applications in August 2014, more than 250,000 visa applications have now been lodged online.
"This initiative has enhanced [our] ability to move the processing of visa applications globally and INZ regularly does this to ensure that visa decisions are made in a timely manner," Scott said.
The agency moves work between different officers depending on capacity and staffing at any particular time, he added.
Scott said it was not possible to provide a breakdown of visas processed offshore or onshore, or how many were processed by offshore officers, as the situation was "fluid and ever-changing".
"All decisions made on visa applications are made by designated and delegated immigration officers so the services are efficient, consistent and of a high standard irrespective of where applications are processed," he added.
But licensed immigration adviser Toni Alexander said applications processed overseas take longer and lack consistency.
"Overseas it can take up to three months, whereas here we get a decision within two to three weeks," Alexander said.
"The offshore visa officers are also not familiar with what's happening in New Zealand, frequently show clear bias."
She also believed the risk of corruption was much higher offshore.