National Finance director Carol Braithwaite was on Friday convicted by a jury in the High Court in Auckland of making untrue statements in a 2005 prospectus.
Two other directors, Trevor Allan Ludlow and Anthony Banbrook, have already pleaded guilty to related charges.
Institute of Directors' chief executive Ralph Chivers says directors should sit up and take notice of the conviction.
"Directors are expected to turn their minds to the decisions they're making and not just accept things that others say at face value, but satisfy themselves that the things they are being asked to sign off are correct."
