Police received a number of reports of possible sightings of Bud, the 71-year-old's giant schnauzer, about 5km north of the township this morning.
A school bus heading for Twizel Area School pulled over after pupils spotted the dog this morning and looked after it until police arrived.
Constable Nayland Smith, of Omarama, said the dog appeared to be in "pretty good'' condition.
"He's off to the vet at 12.30pm to be checked out,'' Const Smith said.
Mr Murdoch has been missing since August 6. He was last seen driving his Green 1994 Toyota Hilux Surf, registration number CHW194.
Const Smith said the search for Mr Murdoch would be expanded this weekend.
The Omarama search and rescue team would be assisted by other SAR teams from outlying areas.
Already more than 480 man-hours have been spent on aerial and ground searches.
Mr Murdoch has declining health and is not mobile without a walking frame.
If anyone has any information on the whereabouts of Mr Murdoch or his vehicle, they are asked to contact their nearest police station.
