You can start work before your IRD number arrives, but you must give it to your employer as soon as you receive it, along with a completed Tax Code Declaration (IR330). Without an IRD number and tax code, your employer must tax you at the highest "no-notification" rate. If you have arrived recently, apply as a "new arrival" — this uses your immigration record instead of extra ID documents. Online applications typically take about 10 working days; paper applications about 12. Source: ird.govt.nz — IRD numbers for individuals, new arrival application.