As far as I know, I don't believe there's any barriers for any northern company to bid on any southern work.
Wally Schumann on Question 214-18(2): Highway No.7 Resurfacing Contract
In the Legislative Assembly on June 13th, 2016. See this statement in context.
Question 214-18(2): Highway No.7 Resurfacing Contract
Oral Questions
June 12th, 2016
See context to find out what was said next.