Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With an Aussie, watch their nerve and the way they move. An Aussie who suddenly growls at visitors they used to tolerate, or who is newly frightened of noise, may be in pain. So may one who is slow to get up or stops jumping into the car. Any seizure, however short, is a reason to contact your vet the same day. A first seizure, one that lasts more than two minutes, or more than one in a day is an emergency: contact your vet now. So is bloat (GDV, a twisted stomach): a swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling. It is most common in large, deep-chested dogs, so skip walks and hard play shortly before and after meals. And if they have been given any medicine and then wobble, drool or seem confused, treat it as an emergency: contact your vet now, and tell them they may carry MDR1.
They'll work out the next cabinet too. Keep the medicines out of reach, and if they get into them, contact your vet now.