FLIGHT OF THE SIMPSONS: What Bret McKenzie and Jemaine Clement might look like on The Simpsons.

Wellington duo Flight of the Conchords are returning to American television with a guest appearance on The Simpsons this year.

It is understood Jemaine Clement and Bret McKenzie rehearsed lines with Nancy Cartwright, the voice of Bart Simpson, over the phone from Wellington before recording in the United States.

Rumours emerged as far back as 2008 that the duo were up for a coveted guest spot, but Simpsons executive consultant Antonia Coffman said then that there were no plans to have them on the show.

But a source close to the pair said they recorded an episode in January that will air later this year.

They had practised lines over the phone with "Bart" before their studio sessions. "They apparently said it was very weird."

Conchords manager Mike Martinovich said he could not make any comment about the appearance.

"I don't know whether there's anything I'm allowed to tell you about that," he said.

It is not known whether Clement and McKenzie will play themselves in their upcoming episode, but it is likely.

The Simpsons, set in the fictional town of Springfield, is the longest-running sitcom on television.

In earlier seasons, most guest stars voiced fictional characters but as the series grew in popularity it has become more common for stars to portray themselves.

Xena star Lucy Lawless and Jurassic Park actor Sam Neill are the only other Kiwis to guest star on the show in its 21-year history. Lawless appeared as herself in Xena costume in The Simpsons' Halloween special in 1999.

Neill was the voice of cat burglar "Molloy" in a 1994 episode.

The Simpsons holds the world record for the most guest appearances in an animated television show, but the producers are picky about who gets to "go yellow".

It has often been said, only partly in jest, that "you haven't really made it in Hollywood till you've been on The Simpsons".

British football superstar David Beckham, now living in Los Angeles, was turned down for a guest spot last year.

Executive producer Al Jean said at the time that Beckham was not famous enough. "I had to phone David Beckham to tell him sorry. I didn't think he was a big enough star for an American audience."

McKenzie and Clement spent time with their families in Wellington this month before flying out last weekend to tour in the US.

Earlier this month they performed sellout shows at Wembley Arena in London.