What is Grabaseat?
Grabaseat is a way by which Air New Zealand Limited can offer ridiculously cheap domestic and international flights without reducing its normal prices.
As stated on their website - "These fares are only available online for one day only or until sold out. Seats are strictly limited - see the online 'counter' for the number of seats left. Travel periods are strictly limited and fares may not be available on all flights or days during stated travel period. Flight restrictions and routing restrictions apply. Prices are per person, child discounts do not apply. General terms and conditions and Smart Saver terms and conditions apply."
The grabaseat airfares are also limited in number so if you don't get in quick you are likely to miss out.