Kylie Liu, a Chinese social media expert and director of cross-cultural consultancy PIN Solutions, estimated that nine in 10 Chinese here who has a smartphone would have a WeChat account.
WeChat has over a billion created accounts with more than 700 million active users including 70 million outside of China.
Massey University China specialist, Associate Professor Henry Chung, said Key's move signalled "the power of WeChat".
The number one social network of choice for Chinese nationals and Chinese in New Zealand has been likened to Skype, Facebook and Twitter combined.
With a built-in web search system, it has the capability of Google's search engine and also the functionality of Apple Pay.
Its mobile wallet function is being used to process more than 500 million payment transactions in 20 countries every day.
"This means the Chinese can follow all messages on WeChat from the PM's office, and the PM can also get first hand information directly from his followers and users," Chung said.
"It will have a huge implication and means a lot for Chinese consumers and organisations."
He said the PM's decision to use Chinese on WeChat would also make his account an effective "crisis management tool".
"The impact from a major crisis can be significantly reduced through direct communication," Chung said.
"More should really follow the PM and explore how WeChat can assist in achieving their business and political objectives."
The China-developed service is one of the largest standalone messaging apps.
Andrew Zhu, a University of Auckland honorary research fellow, has sent a request to follow Key but is yet to be accepted.
"To be really effective, it should be a public account which does not have a limit, and not a private one," Zhu said.
According to Zhu, special approval was required for WeChat public accounts to be set up outside of mainland China.
