Holiday pay calculations have been a grey area in some employers eyes, however the Holidays Act has always specified that bonus and commission payments etc. paid on a regular basis need to be added to the holiday pay calculation.
This article appeared in the Sunday Times over the weekend and may be of interest to you.
http://www.stuff.co.nz/sunday-star-times/business/5675710/Firms-face-back-pay-nightmare
Please do not hesitate to call if you have any questions regarding your leave calculations.