Sydney test-driver carjacks luxury vehicle

Jamie McKinnell and Greta Stonehouse

The owner of a luxury car stolen at gunpoint during a test drive in Sydney's south says his wife is "shaken" because thieves know where they live.

Nash, a 48-year-old relative of the 2015 Mercedes' owner, met the man who'd replied to an online advertisement that listed the car for $120,888.

He was in the passenger seat at a Connells Point intersection on Tuesday night when the driver refused to follow instructions about what route to take.

The driver allegedly pulled out a Glock-style pistol and threatened the man before demanding his keys and phone and forcing him out of the vehicle.

"It was scary because I didn't realise what was going on until he pulled the gun," Nash told reporters on Wednesday.

The car's owner, Raj, said the thief had obviously targeted the car.

"I'll tell you what, my brother-in-law is quite a tough guy but he was extremely shaken," he told reporters.

"It's such an invasion, my wife is really shaken by the whole thing. To have somebody come to the house - now they know where we live."

Superintendent Julian Griffith urged anyone wanting to sell a vehicle to take extra caution when agreeing to test drives.

It was a "concerning" incident for the victim, he added.

"He's distressed and upset, but doesn't appear to be physically injured," Supt Griffith told AAP.

The car is described as having a 4.0-litre engine that can accelerate from zero to 100km/h in four seconds.

The owner was looking for "a quick sale" because he was heading overseas.

The alleged thief, aged in his 20s or 30s, was last seen driving in the car along Homedale Crescent. The Mercedes has the number plate 11SUMO.

Police urge anyone to who sees the car to call triple zero but not to approach it amid fears the offender could still be armed.