They thought she was too friendly to be wild, so decided to take her home with them to Christchurch and get her checked at a their local clinic in Hornby the next morning.
The vets scanned RB's microchip, which was loaded with the McEwan's contact details.
Mrs McEwan was at work when she found out RB had been found and was "in tears all day".
The couple was very grateful Forrester and his partner had taken care of RB when they found her.
"Not a lot of people would do that," Mr McEwan said.
The McEwans did not think RB could have made it from Weston to Glenavy on her own.
They suspected someone had driven her over the bridge, and she had tried to come back but got stuck at the river crossing.
"She wouldn't have crossed the river on her own. The furthest north she has been is the vet in Oamaru," Mrs McEwan said.
RB, who was named after a Nissan engine, had always been a bit of an outside cat, coming inside for a "feed and a scratch". She was adopted as a kitten from the SPCA two years ago and her sister Maloo had been very stressed by her lengthy absence, Mrs McEwan said.
They posted on the Oamaru Lost and Found Facebook page to see if anyone had seen her and held faith she would come home.
Since returning home, RB was "eating like a horse".
"She was really thin. We think she was trying to get food off of people stopping at the rest stop."
The couple said they would like to hear from anyone who knew anything about their cat's journey from Weston to Glenavy.
"We want to try and piece together how she got there."
The incident also highlighted the importance of registering and microchipping pets, they said.
"She could have been adopted out in Christchurch without it," Mr McEwan said.
