MediaWorks has apologised to MoreFM listeners after an expletive-laden rant by a popular night time announcer was mistakenly aired.

The rant by MoreFM Nights co-host Joe Cotton was described by a listener as "foul mouthed" after it aired at 7pm on Tuesday.

One listener told the Herald: "Joe was unaware her microphone was still on and let forth a stream of f**ks and s**ts. She appeared annoyed about being called in to cover shifts without adequate preparation."

MediaWorks spokeswoman Rachel Lorimer said a "small number" of listeners contacted MoreFM after the gaffe.

They were told that the broadcast was of a private conversation between two studios.

"In the middle of it the computer switched the studio live to air resulting in some of the personal conversation being broadcast over the top of a few commercials," a statement from MoreFM said.

"It's a radio announcer's worst nightmare to have a microphone go live when you're not expecting it, and we unreservedly apologise for this.

"We've changed where the studio switching occurs to ensure it won't happen again, and spoken with the announcer concerned.

"No ponytails were pulled in this case, but we absolutely regret this went to air."