exercise is not the answer. He's getting plenty!
Digging reason: this is the solution focused stuff. Why?
Kiss goodbye to the garden. Its now his. And a far less stressful perspective to take from a owners point of view.
Gardens can be redone once pup has matured.
Im for trick training in that 45 mins, as opposed to trying to tire a kelpie x out. kelpie/tired is a oxymoron.
Boredom busting for mine that works:
hide things for dog to find, at various levels of height in garden. eg. in bushes, up trees, down holes, in a dig pit.
Kongs or other puzzles for dogs to solve.
Frozen coke bottles of meat stock tossed into garden as you leave.
Have a dog date for when you are not there, if you know someone with one you can borrow their dog.
Put a platform up, something up high where dog can sit/lay and watch the world.
Buy some bird food (sorry, this is cruel to birds, stop reading sensitive folks) and encourage them to visit the garden to entertain the dog trying to catch them.
Cardboard boxes with 'prizes inside' that the dog has to get into.
Paddling pool with items that sink inside it. Kongs sink.
Paddling pool filled with dirt, with kibble hidden in it.
Basket balls seem to last a day or so, and can be bought at charity shops for $2, but take weeks to tear apart.
Balls or tires hung from limbs of trees
Put add in local shop for some teenager/retiree to come walk your dog after school
All things aside, you've just hit teenager stage. Welcome to the dark side of dog ownership