Highly unlikely you'll need a lawyer for this. What sort of help are you looking for? NZ employment contracts tend to be relatively standardised and pre-vetted by a alwyer to conform with labour laws, so it's unlikely you'll be successful in negotiating the legalese. With respect to salary, fringe benefits, relocation allowances, and so on, I doubt having a lawyer would help -- just discuss the changes you want with the appropriate manager.
Compared to the US (where I'm also from, BTW), NZ is much less litigious. If a potential employee had a lawyer ring me up about an employment contract, now that I've been here a while, I'd be reconsidering if they'd be a good fit.