National leader Judith Collins says allegations surrounding the party's board member Roger Bridge is a matter for the party to deal with internally.
Newshub reported allegations that Bridge called talk-back station Newstalk ZB - impersonating a person named "Merv" - to derail one of the party's Auckland Central candidates.
That candidate - Nuwanthie Samarakone - was not successful in her bid to become the electorate's candidate, with the party picking Emma Mellow last night instead.
Bridge is also the chairperson of the party's Canterbury Westland region. He has not responded to calls from RNZ.
Collins says the allegations are not appropriate for her to deal with, as she is in charge of the party's parliamentary wing, but she does not support such behaviour.
"It's not what we would do, it's not something I approve of," Collins said.
National's Christchurch Central candidate Dale Stephens offered support to Bridge.
He says he had not heard of the allegations but Bridge is "one of the most honourable men" he has met.
"[The allegations] didn't make sense as Roger looks after the South Island," he says.
Stephens declined to be interviewed further.
A senior South Island National Party member says the call to Newstalk ZB was "strange" and noted the similarities in the voice.
"It does sound like him," the member says.
They say the Auckland Central candidacy has been "untidy" but believe the party's overall campaign is looking strong.
The member also says the board does not have the power to elect candidates for the electorate.
"The delegates make the decision, not the board," the member says.
They say the issue has not been raised within National Party circles in the South Island on Tuesday.