Well:
depends on whether or not the firearm is Non Restricted, Restricted or Prohibited.
it depends on what you were doing while on crown land or private property.
It would depend on the permitted uses of the crown land, or whether or not you had the land owners permission while on the private property.
There certainly are circumstances when carrying a shotgun in a backpack while on crown land or private property could be legal.
There are a grocery list of reasons why it might not be legal. You have not given enough information for anyone to give a decisive answer.
In addition to provincial laws which regulate hunting, and what firearms can be possessed and where under the pretense of regulating hunting, there are federal laws for possession, which include places where firearms can be possessed and under what circumstances.
Further there are regulations for the transportation, storage and carriage of firearms.
Lastly there are some very open ended criminal code sections that can be levied against you based on the officers discretion, such as S 86 (1) Careless Use, 88(1) possession for a dangerous purpose ("dangerous purpose" is interpreted broadly and can be anything the Police deem to be inappropriate) and S 90 Carrying concealed (yes, a back pack, even one that met the requirements of the transportation regulations, has resulted in people being convicted of carrying concealed), to name a few.
It should go without saying, but I will say it anyways, that there are pretty much no circumstances under which a shotgun in a back pack can be kept loaded. As silly as it might sound, strapped to the outside of the back pack, or slung on your body, will usually be more legal than in a back pack.
