KATY Perry, recently voted the sexiest woman in the world by Maxim magazine, has tried the celebrity decoy trick at Sydney International Airport this morning.

The singer, made famous by her smash hits I Kissed a Girl and Hot N Cold, posed as the "personal assistant" to her mystery brunette lookalike friend.

The pair touched down in Sydney this morning on a Qantas flight from Los Angeles and emerged with a burly bodyguard pretending to protect the decoy Katy.

Walking behind her friend, Perry hid under her black beanie and dark shades.

Both women wore torn black jeans, shades and matching boots.

While many fans were fooled by the decoy, some worked out the ruse and scored a signature from the real Katy Perry.August 10, 2010 - The star deployed another guise before her flight to Sydney, posting a nerdy photo of herself on Twitter taken at the Teen Choice Awards in California.

Perry, who shared a laugh with her mate after waiting paparazzi photographers snapped the wrong brunette, was surprised by the chilly Sydney weather.

"Ooh, it's really cold out there," said Perry, before leaving in a black car.

There was no sign of Perry's comedian fiance Russell Brand. The 25-year-old has arrived in Australia to perform in Sydney and Melbourne.