Immigration New Zealand says a high volume of applications is largely to blame for a backlog in its visa processing.

Nearly 800 visa partnership applications were declined in the last year, and more than 10,000 are still waiting to be processed.

Spokesman Peter Elms says they're also undergoing "transformational" change, bringing visa processing work from overseas back to New Zealand.

"Doing that at the same time as we're seen an increase in volumes has really meant that we've started to build up a backlog only in certain types of applications."

He told Heather du Plessis-Allan nine out of 10 applicants for a partnership visa get one.

"But we do have to establish that that is a genuine and stable relationship, and that does mean that we have to ask some personal questions to make sure that we're satisfied about meeting that threshold."