Organisers of the Blue Lake Multisport Festival near Rotorua on January 29-30 confirmed Armstrong's entry yesterday.
The seven-times tour de France winner will contest a sprint triathlon, comprising a 750m lake swim, 16km road cycle and 5.5km forest run.
Armstrong became a professional triathlete at 16, and was a national sprint champion in 1989 and 1990 before concentrating on cycling.
He signalled his intention to focus on triathlon and ironman competition at the conclusion of this year's tour de France, when he finished in 39th place overall.
Armstrong is contesting Australia's seven-day Tour Down Under beginnning on January 16, in what is being touted as his last professional race on international soil.
Organiser Charlotte Pearson said the usually low-key event had already noticed an upturn in entries, and fields in both events at the moment would probably be limited to 500 per race.
The event usually attracts around 300 competitors, she said.
Pearsion said event organisers had received confirmation of Armstrong's entry from his media adviser yesterday.
"This is a really big thing for the event, especially since it's our 25th anniversary."
She said the Blue Lake event had been a starting point for many of New Zealand's top triathletes and multisport competitors, including Bevan Docherty, Kris Gemmell and Cameron Brown.
