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It will bring the total number of empty street-level retail and office spaces on Thames St, between Itchen St and Usk St, to 15.
That figure was up from 12 counted in early November.
News of Natural Health's closure follows confirmation another lower Thames St business, Mainstreet Musicworks, will close its doors on April 18 if a buyer is not found.
Natural Health owner Susanne Black, who has run the business for 14 years, said her decision to close was driven by simple economics.
"Honestly, there is just not enough foot traffic and not enough people coming through the door, really. I rely on my loyal customers and that is why I am still here, really.
"The last three or four years especially it's really been noticeable."
She was at a loss to explain why business had dropped off but suspected the apparent retail downturn was not limited to Oamaru, given conversations she had had with sales representatives that visited her store.
"There is money there, but people just don't spend it in town."
Ms Black was not convinced it was a result of online shopping, as has been cited by other Oamaru business owners.
"I don't think it's had a massive affect because 99% of my customers are 65 and over, so they don't really do the online thing."
She believed the Oamaru retail sector had gone through a "terrible" patch in recent times and with the exception of free, all-day parking being introduced and retailers trying to stand out from the rest, did not know what could be done to address the issue.
"You've got to be able to change with the times if you want to stay in retail. You have to do something different from everybody else."
"If someone came up to me and said [they were] opening a retail shop in town I would say do your homework first. It's not the time to start a business."
Attempts to establish an organised retail association did not have a lot of buy-in, and the council should look more closely into what it could do to support Oamaru retailers, she said.
She will continue to run her business from home and offer a delivery service when her store closes on April 30.