"We underestimated this," Bruce Adams, of Adams Flags, said.
Queues out the door, running out of eftpos rolls, and the phone ringing nonstop have been the norm since the start of the Rugby World Cup, he said.
"I have never been involved in a business like it."
Argentinian, English, New Zealand silver fern and Samoan flags were some of the biggest sellers.
Flags ranged in price from $22 to $79. The Dunedin manufacturer was also importing cheaper flags to meet demand.
However, Mr Adams was unclear if flag sales for each country would mirror their final World Cup standings.
Flag poles and bunting were also popular, and orders had been received from all over the country, he said.
"We have had more people through this showroom over the last three weeks than the almost four years we have been here.
"We had a lady turn up the other day and [she] couldn't believe that we could supply a Namibian flag."
Businesses were also buying the flags of World Cup teams to fly on their often bare flagpoles, and would be dressing their shop windows with extra supplies of bunting arriving this week.
Flags were not limited to overseas visitors or New Zealanders buying the silver fern, as local people were keen to show their support for so called "underdog" teams such as Argentina, Georgia and Romania.
Sales before the final game at Otago Stadium between Italy and Ireland were expected to match, or better, those for the first Dunedin match, between England and Argentina.
Mr Adams said he went to the first two matches at Otago Stadium and could recognise his company's flags being waved by rugby fans.
He had also made a St George flag for England rugby player Dylan Hartley to fly on his 1963 Vauxhall Victor, which Hartley bought this month from Mr Adams' father-in-law.
