Pettigrew said the car was a silver, late-model Falcon or Commodore-style sedan and there appeared to be several people in it.
"I yelled out - 'I've got your rego'," he said.
The car then drove off, with the left-hand back passenger door open, leaving the offender behind.
The would-be thief dropped the carpets onto the footpath and ran across the road, in front of oncoming traffic, disappearing behind The Barracks building.
Pettigrew said "I wanted to follow him, but I had my arm in a sling - I've just had shoulder surgery - and I also wanted to get my carpets because it was raining."
The carpets were recovered with no damage, and police were alerted to the incident.
In his six years with the store, Pettigrew says he has had no other trouble such as this, apart from a recent report from a customer who said two women had been seen carrying carpets across the Guyton St intersection and away down St Hill Street toward Cooks Gardens.
That incident prompted Pettigrew to secure the outdoor displays, "and what I put in place seems to have slowed them down, but I'll have to find ways to make it even more secure".
"I'm not giving up putting stuff outside because of these people," he said.
Whanganui police searched surrounding areas but have not yet located the offender.