His brother Simon says while there will "obviously going to be all sorts of theories" about the disappearance, the family is focussing on any sightings of him or his blue 1994 Subaru Legacy station wagon.
In a Facebook plea for information, Simon said: "Very unlike him to go walking - has never done so before and he has been in good spirits."
Detective Inspector Darryl Sweeney, who is leading the investigation, said police "have concerns for his welfare".
It remains a missing persons investigation, police say.
Missing person posters have been placed around the area.
One neighbour said police came to her last Thursday wanting to eliminate her silver car from their enquiries.
"It's really strange. It has us all worried," she said.
McGrath's blue station wagon was taken away from his property - just a few doors down from 'Black Widow' husband killer Helen Milner fatally-poisoned Phil Nisbet in May 2009 - at the weekend.
Now, Sweeney is appealing for sightings of the Subaru over the weekend of May 20-22 in the Halswell area.
Neighbours describe McGrath as a quiet man who rarely had visitors.
Anyone who has information about McGrath or has employed him in the last few weeks is asked to call Christchurch police on 03 363 7400 and cite file number 170523/6008.
Information can also be provided anonymously by calling Crimestoppers on 0800 555 111.