The Department of Labour just released new guidelines for those who deal with contractors. A handy read if you are involved in using contractors to do work at your place. http://www.osh.govt.nz/order/catalogue/contracting-health-safety.asp
There are examples given right from how to select the contractor and allocate work.
Section 18 of the Health and Safety in Employment Act 1992 creates a duty for principals to contracts that applies to these situations, and this guidance has been prepared to help businesses meet that duty and has been signed off by Lesley Haines, the Acting Deputy Secretary of Labour, Workplace.