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However, prices fell for other fruit and vegetables, such as mandarins and broccoli.
The average price for a 200g avocado rose to $5.06 in May this year, up 37 percent from $3.69 in April, Stats NZ said.
The price in May this year, up 50 percent from $3.38 at the corresponding imte last year, is the highest for avocados since the series began.
Stats NZ prices avocados per kilogram, and the latest movement reflected both higher prices and smaller fruit.
Seasonality impacts avocado prices, Statistics NZ's prices manager Matthew Haigh said.
"Typically, avocado prices peak in July and August, as the main harvest season is from August to March," he said.
According to the New Zealand Avocado Growers Association, harvest volumes in the year to June 2018 were down to around half that for the previous season.
This decrease affected the average retail prices of avocados in the past 12 months.
Overall, food prices remained flat in May, falling 0.1 percent after seasonal adjustment. Falls in fruit and vegetable prices were offset by rises in meat, poultry, and fish prices.
Food prices decreased 0.1 percent in the year to May 2018, influenced by lower fruit and vegetable prices.
Despite the overall price fall, there were increases in restaurant meals and ready-to-eat food prices (up 2.9 percent), and non-alcoholic beverage prices (up 4.6 percent).