The public are getting behind their favourite flag finalists causing merchandise associated with the designs to sell out.

Kyle Lockwood designed two of the four finalists - the red, white and blue flag with the silver fern and southern cross, and the black, white and blue flag with the silver fern and the southern cross - and he's been inundated with requests for merchandise.

Chris Mullane, a spokesman for Lockwood, said the buying rush was unexpected.

"There has been a considerable number of requests, on a scale that hadn't been anticipated, hence the items' selling out."

He said demand was highest for items depicting to the black, white and blue flag.

He said now that people have seen the final four designs, they are wanting to get behind their pick of the bunch, and the black, white and blue flag seems to be a firm favourite, according to sales.