The Auditor-General has recommended an investigation into Maggie Barry's alleged use of parliamentary staff for party purposes.
In December 2018 an official complaint was laid over allegations that the North Shore MP instructed a staff member to carry out National Party work during work hours.
A spokesperson for Ms Barry said on Thursday she "denies any breaches occurred and welcomes the opportunity to talk with Parliamentary Services about any concerns".
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On Thursday Auditor-General John Ryan announced in a letter to Parliamentary Services that "further investigation into the concerns is necessary".
"After discussions with staff from your office, we have decided that the most appropriate course of action is for the Parliamentary Service to carry out that investigation."
The Auditor-General may decide to get involved again with the investigation at a later date.
In December Newshub saw an email from Ms Barry instructing a ministerial staff member to work on a letter to National Party members. The email was sent during a work day to a parliamentary email address.
At the time, electoral lawyer Andrew Geddis said it appeared to be a breach.
"When you have an email like this sent during work time, it looks very much like they are being asked to do it on the taxpayer dollar - and that's what they are not allowed to do."
The National Party at the time refused to discuss the email without a seeing a copy of it, but Newshub had agreed with its source not to share the email.
Ms Barry stood by her denials, saying it was difficult to deal with anonymous claims.
National has been approached for comment.