On a daily basis, as you go about life in New Zealand, you'll pay Goods and Services Tax (GST) of 12.5% on just about everything you buy in New Zealand.  Most price tags you see in shops always include GST.

The only things that don't have GST added are financial services, and the rent or purchase price of residential property. 

The New Zealand Tax Year
New Zealand's tax year runs from the 1st of April, to the 31st March.

New Zealand tax rates
The New Zealand tax system is considered one of the simplest in the world rates are:

• 19.5% income up to $38,000
• 33% for income from $38,001 to $60,000
• 39% for income of $60,001 or above

The official link to working out your tax amount and code is here, and for individuals here.

ACC Levy
In addition to the above income tax, you will also pay 1.2% of your wage for ACC (accident insurance). 
This levy covers the cost of medical treatments in the event that you have an accident.

KiwiSaver - New Zealand's work-based savings initiative
Since 1 July 2007, most New Zealand residents and people entitled to be in NZ indefinitely (who are 18 years or older) are automatically enrolled in KiwiSaver when they start a new job.
Wikipedia states that employee participants can choose to contribute either 4% or 8% of their gross pay, and can switch rates at any time.

You will need an IRD (Inland Revenue Department) Number
As you can see from above, you will have to pay tax on all income you receive while in New Zealand (there is no minimum). 
This means that that before you apply for any job, large or small, you will have to apply for an IRD number from http://www.ird.govt.nz. 
You will be asked for it -- so don't wait till you are. Once applied for, you should get it within a few days.
Info on how to easily apply for the IRD can be found here.

What it is
As you can see from above, you will have to pay tax on all income you receive while in New Zealand (there is no minimum). 
This means that that before you apply for any job, large or small, you will have to apply for an IRD number from http://www.ird.govt.nz. 

You will be asked for it -- so don't wait till you are. Once applied for, you should get it within a few days.

How to get an IRD Number
Info on how to easily apply for the IRD can be found here.

If your income is from a salary or wages, your tax will already have been deducted under the Pay-As-You-Earn (PAYE) system.
This means that the pay you actuallyr receive has already been tax deducted.

If your employer provides you with non-cash benefits of some kind, as part of your employment package (eg: use of  car, etc) you will not be taxed on these benefits: your employer will already have paid a Fringe Benefit Tax on them.

Finally, if you are earning income from your own business, and it has not been taxed via PAYE, you'll generally have to pay your provisional taxes in 3 installments during the year. When you file your tax return at the end of the financila year, the provisional tax you've already paid will be offset against the year-end