The award is presented annually by the New Zealand Association of Event Professionals, and Queenstown Winter Festival was named the winner.
Best-established event recognises an event that demonstrates innovation, rejuvenation, initiative, economic, environmental, social and cultural sustainability.
The judging panel comprised "industry experts" across various event disciplines throughout New Zealand.
Festival director Sally Feinerman, who has managed the festival for the past six years, accepted the award.
"I am absolutely delighted that the festival has been given due recognition after 35 years.
"This is a culmination of support from the key stakeholders, the Queenstown community and the festival team as a whole.
"Essentially, the American Express Queenstown Winter Festival is an iconic event that heralds the beginning of winter to Queenstown, New Zealand and the world, and it certainly ticks all the right boxes when it comes to the selection criteria."
Destination Queenstown chief executive Stephen Pahl congratulated the festival team on delivering a "world-class event".
"This year's festival has been widely applauded as the best to date - so it is only fitting that this achievement is recognised at a national level."
Dates for the 2010 festival have been confirmed as June 25 to July 4.
