In July 2018 they dropped 22 per cent, netting a saving of $179, and in December a whopping 44 per cent, saving $638.
Among the other months of 2018, the largest price drop was 11 per cent, with an average drop across those months of just 5 per cent.
Across the top-10 destinations out of New Zealand in 2018, the best-value time to book was on average two and a half months in advance.
House of Travel commercial director Brent Thomas said late-summer and autumn was "most definitely" a sale time of the year for airlines.
"People go away at Christmas time and think about the year ahead. Often it's not until February they start to come to House of Travel and start booking their trips for 2019," Thomas said.
"January, February, March are incredibly busy for us, and typically we would do 50 per cent of our sales in that period. It's a key selling time for the airlines. "
Thomas said in general the price of international outbound flights had seen a "steady decrease" over the past five years.
Based off the Kayak stats, the average price of a flight to New Zealand's top-10 international destinations for a return economy seat was $120 cheaper in 2018 compared to 2017.
A major reason for this was the influx of new international carries entering the New Zealand market in recent years.
"We're only 4.5 million people but we've got nearly 30 carriers in New Zealand," Thomas said.
Another major factor Thomas cited in decreasing Kiwi flight prices was the increased capacity, hence frequency, of most airlines' outbound routes from New Zealand.
There was also a "real drive" for some carriers to invest in fuel efficient aeroplanes.
"You take the likes of Air NZ, they have said in the past that 30 per cent of their operating cost is the fuel," Thomas said.
"So when you get a plane that is between 20 and 25 per cent more efficient, and fuel accounts for 30 per cent of their costs, automatically you have a reduction in their operating costs."
In the past 12 months, Qantas has offered an average of three sales each month for flights from New Zealand to Australia.
Added to this, their subsidiary company Jetstar averaged one sale each month on New Zealand flights.
Qantas executive manager sales and distribution, Igor Kwiatkowski, said there were still broad trends across the year for cheaper New Zealand to Australia flights.
"Travellers can also take advantage of good value fares when flights to Australia are less busy over winter, but for peak periods over summer and school holidays our advice is to always book as early as possible," Kwiatkowski said.
Thomas said the frequency of Kiwi flight sales was rapidly increasing, saying it was a reflection of the growth of the local aviation industry.
"New Zealand's population is only growing marginally, yet last year the passenger growth for New Zealand outbound [flights] was 8 per cent," he said.
"New Zealanders are definitely opening their wallets for travel."
Best Time to Book Flights to Top International Destinations from NZ:
1. Sydney - 2 months in advance, cheapest month to travel September
2. Melbourne - 2 months, cheapest month March
3. London - 3.5 months, cheapest month July
4. Nadi - 3 months, cheapest month October
5. Los Angeles - 3 months, cheapest month November
6. New Delhi - 2 months, cheapest month March
7. Honolulu - 2.5 months in advance, cheapest month March
8. Bali - 1.5 months in advance, cheapest month November
9. Brisbane - 3 months, cheapest month March
10. Gold Coast - 2 months, cheapest month February
