Some nail salons are accused of duping customers by filling expensive-looking polish bottles with cheap brands.

One international company is sending "mystery shoppers" to get their fingernails painted, then forensically testing the clippings to prove the allegations.

Terri Grace, sales and marketing manager for Global Beauty which distributes OPI nail polish, confirmed the company was about to send letters to about 50 nail salons - most based in Auckland - warning they could be caught for "fraudulently" using their products.

Many of the salons are run by foreigners and the letters will be translated into about six languages, she said.

The letters would inform salons that the company knew about the problem and would be taking further action if they did not stop using the cheaper products.

Mrs Grace said the company had suspected it had been going on for some time, but had recently had an upsurge in complaints about its products, which retail at about $30 a bottle.

Forensic tests returned in January confirmed their suspicions that the nail polish was not OPI's.

One customer, who sent a letter to the company, said she was shocked to watch a man at an Auckland nail salon fill several OPI bottles with acetone nail polish remover to clean them out.

"I then saw the man walk away and come back a few minutes later with a bag full of nail polish which he had brought at the $2 shop next door," the letter read.

The woman, who took photos of the man and sent them to the company, complained the manicure she received was "very average and the polish started chipping the very next day".

Mrs Grace said the company was fielding similar complaints from customers and was concerned about the damage to its brand.

"It's a big problem ... It [the cheaper product] doesn't last as long so obviously people are getting a little bit upset about it.

"We have actually taken on the services of a forensic analyst," Mrs Grace said. "What they can basically do for us is, we can go into a nail salon, we can have our nails painted and cut a piece of our nail off and send it to them and they'll be able to analyse it and see if it's OPI."

OPI nail polish has unique names, such as "Suzi Say Da!" and "I'm Not Really a Waitress" and customers who knew what the colours looked like would recognise the cheaper colours, Mrs Grace said.

In one case, a colour that should have been red, came out as gold.

It was upsetting because nail salons using the proper product were also having their reputations tarnished.

"They get the rap for it too."

The Commerce Commission has been informed, as has the cosmetics industry's governing body, the Cosmetics Toiletry and Fragrance Association. The problem is believed to be occurring in Australia as well.

Anyone wanting to check they are receiving original OPI products should check a salon for a sticker that says the store is an authorised salon.

A Commerce Commission spokeswoman said only the courts could decide whether the Fair Trading Act had been breached and set appropriate penalties, but anyone could take their own action.