Paul Whiston, Paymark's head of sales and marketing, said nationally there were more than two million transactions and $120 million in sales through its network, $14.2 million up on Boxing Day last year.
Mr Whiston said this was the first time in five years there had been double-digit growth.
The popular sales items included computers and phones (sales up 31.7 per cent), clothing (up 20.7 per cent) and jewellery (up 24.4 per cent).
And he said department stores had enjoyed big increases in sales, probably driven by the Boxing Day sales.
Wanganui retail sales were better than a number of other provincial centres, including Bay of Plenty, Marlborough, West Coast and Southland.
Daniel Clark, store manager of The Warehouse in Trafalgar Square, said Boxing Day this year was one of its best trading days of 2012.
"There were people outside the doors when we opened at 8.30am and we had a constant stream through here all day right up until closing at 8pm," Mr Clark said.
"This was definitely one of our best trading days of the year," he said.
While the Wanganui store had benefited from the foot traffic generated by the Cemetery Circuit motorcycle races, there was more behind the upsurge in sales, he believed.
"There is obviously more discretionary spending happening this year than last and that's good for local retailers," Mr Clark said.
And Farmers Trading Co's department store and its home store, both on the Avenue, recorded excellent trading days.
Store manager Kirsty Johnson said the hot weather got a lot of people out shopping and the cool of the air-conditioned store brought them inside to shop on Wednesday.
Ms Johnson said the special offers the store had running until Sunday had been an obvious attraction.
"Womenswear has been busy and the toy section is always popular, with kids getting money for Christmas," she said.
Yesterday's trade continued the trend.
"We were relatively quiet in the morning but we've been constantly busy throughout this afternoon."
Ms Johnson said considering the number of pre-Christmas sales a number of city retailers had held this year, the Boxing Day turnover "wasn't too bad".