Retail sales declined a seasonally adjusted 0.3% to $5.5 bilion in April, led by a decline in spending in department stores, official figures show.
Excluding spending on cars and fuel, core sales edged down slightly, reversing a 1% gain in March.
Statistics New Zealand says the trend shows core sales have fallen gradually since October last year - the first decline in 15 years.
Economists say weak retail sales indicate consumers are cautious about their spending and are concentrating on paying off debt.
However, the fact that school holidays fell entirely in April and that Easter came later than usual boosted spending on accommodation in the period by 7%, the highest monthly increase in seven years.
Despite the decline in the monthly figure, retail sales are still up 2.4% on the same period a year ago.