There are now 15 K-mart stores in New Zealand and the first opened in Auckland in 1988.
The Hastings store followed four years later, opening on October 20, 1992, and remained one of the "top five" in the country, Mr Wynyard said.
"It is definitely a milestone for the company to have this store reach 20 years.
"We still have about 12 or more staff that were originally here when the store opened as well as some other long-timers of about 17 to 18 years.
"We don't have a lot of turnover and most of those who leave go to pursue opportunities overseas or in other regions."
Mr Wynyard said the store had planned a series of events this weekend to celebrate the birthday and hoped past and present staff would come along to share the anniversary.
K-Mart was reported to be struggling about a decade ago but Craig McKeown, a New Zealander previously working with K-Mart in Australia, returned to his home country in 2004 to lead the recuperation.
At the time he said there were several issues that needed to be looked at, including merchandising, operations, and advertising. The New Zealand arm also needed to become less Australia-centric.
Customers also noticed changes at the Hastings K-Mart store over the years as it evolved to keep pace with the challenges of competitors and the tough economic climate.
"I think like anything life changes and in the last three of four years we have seen a huge shift in culture and leadership since Westfarmers Ltd took over from Coles Myer.
"I believe we have led the change in the market but not following our local competitors and doing something a little bit different.
"We looked at making shopping better for families, improving the way we deliver our products."
The strategy would be used to keep driving K-Mart past the current difficult trading times.
"I believe it will stand us in good stead looking ahead to the next 20 years as we are planning to be around for quite some time in the New Zealand market."
There are now 185 K-Mart stores across New Zealand and Australia.
The company employees more than 25,000 people across both countries.