An analysis by consumer group Choice of 95 drinks from outlets such as Boost Juice, Donut King, Wendys, Gloria Jean's and New Zealand Natural has found 81 of them are high in sugar.
"Smoothies might have a healthy image, but some are packed with hidden sugars," said Choice spokesperson Ingrid Just.
Drinks which contain ingredients such as high-fructose syrup, fruit juice concentrates and artificial flavours and colours are often higher in sugar than actual fresh fruit, she said.
"This makes smoothies more like a sugary meal than a snack."
The review found some Baskin and Robbins yoghurt smoothies had between 29 and 31 teaspoons of sugars.
It also found that Boost Juice's super smoothies, while made with real juice, are high in energy and close to 2000 kilojoules per regular serve, roughly the same number of kilojoules as a meal.
According to dieticians, snacks should contain about 600 kilojoules.
The study also highlighted a tendency to supersize regular or medium serves and inconsistency between serving size.
At Donut King a regular is 280mL, while at New Zealand Natural a regular drink is 650mL.
The consumer group said people should consider all the ingredients in a drink before making a choice.
"If you are an active person and you want a quick pick-me-up, some outlets have better options with smaller sizes, fewer kilojoules or less added sugar," Ms Just said in a statement.