Btech and welcome.
If you don't have either PR, a long term Work Permit or any other long term visa, you do have to pay GST of 12.5% on importing your personal possessions. If you have a job offer that you intend to take up, it might have been better to apply for a WP instead of the WHV and you can progress on to a WTR later if you wish or now if you decide you want to go straight for that type of visa.
The WHV is an easy visa to get but it won't be that helpful if you want to work full time or if you want to bring in your personal possessions. A WP isn't difficult to get either so I don't know if you can switch from a WHV to a WP. It may be that you would have to enter NZ and then apply for a WP before you take up your job offer but it might be worth asking if you can now apply for a WP and bin your WHV. More expense but certainly not as much as it'd cost you to bring in your stuff.