DEAR demented.

Rockhampton's Paul Madsen hadn't even gotten to the body of his letter from nbn and he was already seeing red.

The Rockhampton businessman opened the letter early Wednesday to discover instead of addressing it to his name, they wrote to "De Mented."

After a string of Rocky nbn catastrophes for businesses in the Rocky region, Paul's unbelievable experience only adds fuel to the ongoing fire.

The purpose of the letter was to remind the Koongal man to make the switch to the national broadband network as time was running out.

A spokesperson for nbn yesterday apologised for the error.

Paul, who runs Madsen Gardening Service, thinks it was all part of a sarcastic joke.

He said he would never do anything like this in his line of business, saying it was unprofessional for a reputable company.

"If I sent a letter like this out to my clients I would go out of business," he said.

"I don't mind getting the reminder to switch but this was really unacceptable."

Paul, who has been a Rocky local for more than 40 years, said he had never received a letter quite like this howler.

He believed the act was intentional of someone trying to "stir the pot" since he was yet to switch to the provider after he heard a string of bad experiences.

An nbn spokesperson yesterday said the company "wholeheartedly apologised" to Paul for any embarrassment or hurt caused by the letter.

"Recently nbn has started to personalise the letters we send out to people as the network rolls out in their area," a spokesperson said.

"Unfortunately in this case our external vendor, despite having more than 70,000 entries of salacious and inappropriate terms in its suppression file, did not pick up on this term.

"We have reported the issue to our vendor to help them ensure this doesn't happen again."