The "price promise" means that if the fare booked for their holiday is reduced, passengers booking early receive on-board perks up to the value of the reduction.
Singapore Airlines also offers a price guarantee on its early-bird fares to the Northern Hemisphere. Customers who book in October or November know they won't be advertised at a better rate later.
But Friend said it did not work well for the airline.
"Every other airline undercuts them, and they can't lower the price. It ... sometimes goes wrong and they are left with empty flights to fill."
On Air New Zealand website grabaseat, Friend said customers would sometimes notice a fare advertised at one price would later be available at another. The cheapest fares were usually for flights three to six months away but sometimes people could find them getting cheaper as the date neared.
He said airlines would offer deals to get money in early to help with cashflow - but then if there was a soft week or month approaching they would offer further discounts to fill seats.
One traveller, Richard, who did not want to be identified, booked tickets six months out for a family function in Christchurch. At the time, there were only two of the cheapest seats available and the family had to pay more for extra seats.
"Then just a month ago we had to book an additional seat and it was the cheapest of all the seats. I thought then that was not a good look and now today we had to book another seat and it also was at a cheaper rate than we had to pay six months ago. They advise on the web all seats sold, but they are not."
An Air New Zealand spokeswoman said customers could set up alerts to let them know when a particular fare was available below a certain price. The website offered advice on the cheapest times to fly.
Brad Monaghan, of Trade Me-owned Travelbug, said the past six months had clearly shown the trend for people to hold on for a discount when booking. Most people were booking within two weeks of travel.
Friend said his company did not have a policy on what advice to give customers about booking early or waiting.
"But we would advise that if you know the exact dates you want to go and they are available now and you are happy with the price, book it."
He said in the worst cases, if the price had dropped a lot, it could sometimes be cheaper to cancel a booking, pay the cancellation fee and rebook.
