The electronic payments provider's December data shows spending through its system topped $6 billion, but the value of goods and services grew only 1.2 percent for the year, compared to a five percent growth rate for the previous 11 months.
Paymark's head of marketing Darren Hopper said the low annual growth rate was well below the rates seen in recent months.
"We also saw widespread discounting, which appears to have accelerated the trend towards lower average transaction value."
The average transaction value for the month was $49.77, down 0.8 percent on 12 months earlier.
Mr Hopper said the numbers suggest retailers are under pressure, from things like the growing use of scheme credit and debit cards adding to merchants' bank fees, and e-commerce.
"Anecdotally there were also many sales, starting back on Black Friday, and this may have pushed the average transaction value lower as well, although this is difficult to see in Paymark figures as only the total transaction value is recorded. There may have been more, or fewer items than usual within each transaction as well."
Toy retailers, hardware and home decorating stores and chemists all recorded higher spending compared to the previous December, while retailers and providers in the accomodation, electrical and electronic, and clothing and footwear areas all saw a drop.
The busiest day of the 2018 year was December 20th, with $296 million worth of transactions processed.
This story originally appeared on Radio New Zealand