"There have been several contributing factors impacting monthly spending over the past four months," Whiston said.
"In January we saw a high 5.1 per cent year-on-year increase, exaggerated by January 2012 having an extra Sunday than January 2013, typically a low spending day."
February annual spending was down 1.8 per cent because a leap day in February last year gave an extra day of spending, he said.
March growth was also below average because Easter fell in that month of April this year. April saw strong annual growth of 5.9 per cent.
"Balancing the results over January to April levels these seasonal effects to reveal an annual growth of 3.9 per cent, up on the 3.3 per cent average of 2012; so a positive start to the year for retailers in general," Whiston said.
The Canterbury recovery was helping drive the higher national average growth rate, Whiston said. For the first four months of the year, spending in the region was up 6.9 per cent compared to the same period last year.
Other regions to experience growth included Otago and Waikato (+4.5 per cent) Marlborough (+5 per cent) and Hawke's Bay (+4.9 per cent).
Auckland/Northland's spending growth of 4 per cent was above the national average but only slightly above the 3.9 per cent growth averaged in the region last year.
Some retailers in particular have enjoyed accelerated spending growth in the first third, with food and liquor spending up 4.1 per cent, pharmacies up 6 per cent and building and hardware up 9.6 per cent.
Compared to April last year, spending in hardware stores has increased 10.4 per cent, appliance shops 10 per cent and furniture and floor covering outlets 12.9 per cent.
The number of card transactions was 6.2 per cent higher last month than a year ago. Credit card usage rose 10.5 per cent and debit card usage 4.9 per cent.