In the latest IRD agents answer the IRD provides some further commentary on this issue:
The attribution rule is an anti-avoidance rule to prevent employees using an intermediary (usually a company or a trust) to avoid paying income tax at the top individual marginal rate. When the rule applies, the income of the intermediary is attributed to the individual who provided the services.
http://www.ird.govt.nz/aboutir/newsletters/agents-answers/2010/agents-answers-issue122-03-10.html#08

In your comment made 28th Jan, are you implying, if you contract through using a company, you are still liable as an individual and not company rates,if you work more than 80% of the time for one party?

Cheers

Evan

When using a Contractor Management Company such as CXC Global, you to become a PAYE employee for tax purposes. Under this model you will generally be entitled to salary packaging and more complex payroll benefits, such as living away from home allowance.

Salary packaging provides a way of structuring your income so that you can purchase business related tools and services in pre-tax dollars – as you would if you were an independent contractor. The result is that you gain the tax advantages of being self employed, but do not have the hassle of setting up or running your own structure. You will not have to pay additional accountants fees and if you are with CXC Global for 6 months or more they pay for your annual tax return.

They do charge a small percentage based fee for this service, however this is typically covered by the tax savings they generate for you. The result is reduced tax, increased pay, increased free time and virtually no administration.

For more information on this contractor management solution please see http://www.cxcglobal.co.nz, or email info@cxcglobal.co.nz, (09) 358 4133.