Paymark's head of sales and marketing Paul Whiston said card spending was continuing to fluctuate, moving up one month and down the next.
"Part of the see-saw pattern is due to the changing number of Sundays in the month, partly it is due to changing petrol prices," he said.
The volatility was also due to a comparison with a unique period last year in which the country was leading up to the Rugby World Cup.
That meant accommodation spending was down between September 2011 and September 2012.
Hospitality spending was $376 million last month, 0.4 per cent lower than August and 0.6 per cent lower than a year ago.
When these one-off factors were stripped out, the underlying growth rate remained around 3.3 per cent per annum for the September quarter and similar for the month of September, Whiston said.
He described that as "a modest growth rate but spending growth nonetheless".
Paymark measures spending across four sectors; housing, discretionary, non-discretionary, and hospitality.
Consistent with housing market figures, New Zealanders are continuing to spend more at the likes of home decorating shops, floor covering outlets and gardening stores.
A total spend of $309 million in this sector was 0.7 per cent higher than August and 8.2 per cent higher than a year ago.
Non-discretionary spending, including supermarkets, meat and fish vendors, petrol stations and power companies, was $1.491 billion in September.
That was down 0.3 per cent from the previous month and 4 per cent higher than a year ago.
Discretionary spending, which included the likes of travel companies, pharmacies, gyms, and doctors, was $517 million. That was up slightly from August, by 0.4 per cent, and 1.1 per cent higher than a year ago.
The Paymark Consumer Index, which covered all electronic transactions through the Paymark network, showed seasonally adjusted card spending slipped from 1051 in August to 1046 last month.
Figures out from Statistics New Zealand last week showed New Zealanders' overall credit and debit card spending declined 0.6 per cent between August and September, to $5.6 billion.