Here is the boundary map for the CRC Mid-Canterbury/Opakihi Constituency. This incorporates the Selwyn District portion of Greater Christchurch.
The markers on the map are railway stations and mileposts, so you can see that it incorporates parts of the MSL, Hornby line, and Midland Line up to Arthurs Pass.
Here is a closer view of the boundary with Christchurch City along the rail corridors.
This shows the Hornby line corridor to Lincoln and the Main South Line corridor to Rolleston.
A similar map to the previous one, but showing the current bus routes that are operated by Ecan in Greater Christchurch, that terminate in Mid Canterbury. These are Yellow Line to Rolleston, Route 80 to Lincoln, and route 820 from Lincoln to Burnham.