The 18 month countdown? Do you mean, once you have the offer, you need to send in your passport within 6 months for the stickers, and then activate the visa within a further 12 months?
If that's it, then no, they don't waive it. However, it's not as restrictive as you seem to think. The stickers (for each family member) MUST be activated by arriving in NZ within a year of the date they are granted, or they'll lapse and the whole thing would have been for nothing, but if you haven't finished up your affairs in your home country, you could if you wished get straight on the next plane out. You would need to sort yourself out to get back to NZ ready to stay and start your life there within the 2 years' life of the travel conditions.
I'm puzzled - if you have a job offer, how long are you expecting the employer to wait for you?