She also did not carry out a skin assessment, despite the teenager's skin being "very fair", with no base tan.
The goggles provided to her lacked an elastic strap, which meant they did not have a tight seal, and ultra-violet (UV) light would have been able to penetrate her eyes.
The teenager did not use the sunbed, but waited in the room for 12 minutes before going out to pay $12 for the service.
When an enforcement officer visited the premises the same day, McCarthy said she only asked for proof of age if a customer appeared to be under 18.
She told the officer "I have no excuse. Clearly I should have checked".
Brownlie said sunbeds exposed users to higher levels of dangerous UV radiation than the sun, and increased the risk of melanoma and other skin cancers.
There was "no safe limit" for their use.
Epidemiological studies showed the risk of melanoma was increased by 75 per cent when sunbeds were used before the age of 30, and sunbeds increased the risk of melanoma by 20 per cent overall.
Their use was also associated with increased risk of melanoma of the eye.
The ministry's sting operation involved checks of 22 premises, of which two were alleged to have provided services to a person under 18.
The legislation allows for a fine of up to $2000 for an individual, and $10,000 for a company, for failing to comply with the sunbed regulations.
Noting the defendant was not in court, and her appearance would have "shown some contrition", Judge John Brandts-Giesen said she had acted carelessly by failing to check the customer's age, and by not having a consent form for her to sign.
"Tanning services have an intrinsic and real risk of causing melanomas and skin cancer.
"The defendant in this case would've been fully aware of the dangers, and the regulations."
After a 25 per cent reduction for her guilty plea, he fined McCarthy $600, with prosecution costs of $130 and court costs of $130.
McCarthy declined to comment when contacted yesterday.
