More than a quarter of the 600 weighing instruments tested don't comply with regulations, New Zealand's weights and measures watchdog, Trading Standards, says.
Metrologist Davis White is part of a small team at Trading Standards responsible for ensuring equipment and goods manufactured, packed or imported into the country are exactly what they say they are.
"To make sure that consumers are getting a fair deal and also to make sure that businesses are trading fairly," he said.
Food and Grocery Council chief executive Katherine Rich said the law around weights and measures was vitally important for fair trading in this country.
"Weights and measures sets up how stores will calibrate their scales. It underpins how food manufacturers pack all their goods and without it consumers wouldn't be assured of what they were getting, she said.
Horticulture New Zealand chief executive Mike Chapman agreed it was important consumers got what they paid for.
"The tendency would be to over fill a little bit, as opposed to under fill, because under filling you're not giving the value you should be giving. That's not uncommon, but the aim of course is to give value for the money paid," he said.
Petrol pumps attract the largest number of complaints, but are the least likely to be upheld, with only one faulty pump found last year.
But Mr White said he did not think people were breaking the law intentionally.
"There are a great number of small-to-medium-sized businesses and they grow quite rapidly from very small, maybe even a one-man band, to say five employees and then 10 employees.
"They might not have the correct equipment from day one and the fact that they don't come through our front door to get that advice straight up is a problem for us," he said.
Ms Rich said she was aware of that issue.
"Most food businesses in New Zealand are small businesses and that means the person who runs that business is wearing lots of different hats; they're the marketer, they're the sales person, they're the general manager and then they're also the person in charge of regulations.
"Weights and measures can be quite complex, but I think some of the work that [the Ministry of Business, Innovation and Employment] has done over the last few years, they've tried to demystify what some small businesses have to do.
"But you still have to take the responsibility ... because if you don't get it right then you can find yourself in trouble."
Trading Standards prefers to educate businesses rather than punish - in the last financial year most problems were sorted out with a warning letter.
Only a handful had to be issued with an infringement notice and no one has been prosecuted.