It depends what sort of knife you're planning on taking mate.
If its less than 3" long and doesn't lock then it's legal, you do not need a reason. If it is longer than 3" and or locks then you need a reasonable excuse, this is a subjective call though. As it is subjective, if you were stopped it would depend on whether the police officer who stops and sees you with it/finds the knife (search powers are a whole different thing) thinks your justification is 'reasonable'.
If you are in the wilds it is
A) unlikely you'd be stopped by the police
B) unlikely you'd have a problem if you are stopped
But, if you do get stopped and the police officer doesn't accept your justification you run the risk of being reported/arrested for carrying a pointed/bladed object or an offensive weapon (depending on the knife)