Members from Australia want to chime in with their experiences?
Don't forget that it's more than just four-legged predators to be aware of. Humans have been known to steal guinea pigs right out of hutches or even take the pigs out and harm or even kill them. Local dogs or cats can also be drawn to hutches. Don't forget about cold snaps or heat spikes. Spiders, flies (fly strike is no joke!).
Also, in many cases, although not intentional, many guinea pigs left outside are out of sight, out of mind. The intention is to go out and check on them, to feed them, etc but without the visual inside, sometimes it can get forgotten. Also, with them outside, you can miss signs of illness that you would notice if you were walking by them while inside. Guinea pigs are prey animals so they instinctually will hide their symptoms until it's almost too late. Guinea pigs can go from seeming perfectly healthy to dead in 24 hours. So time is of the essence when it comes to noticing symptoms.
I think I know my area? The living area where I spend most my day has wide windows and sliding doors where I can perfectly see my pigs so somebody stealing them isn't of concern, they are also not far from the door either so I can see them popping their heads up. And I know about fly stroke which is why I'm investing in a FLY screen to cover the cage, as stated in it's name flies or any other insect cannot get through it unless it is intentionally cut.
I'd also like to add on the topic of them being stolen, I have a 7 month old puppy that is absolutely obsessed with my pigs so much that he growled at me once without realising who I was when he heard me entering the room where my one pig (at the time) was being held. The cage they are in is raised when they are outside where my dog (who is tiny but possessive) cannot reach or touch it in the slightest, I also do not let him anywhere near them when they are out of their cage. He is constantly growling and barking at people or any type of bug or animal that get to close to the house that I have to check up on him and who he is barking at whenever he does so.
So not only are my pigs in my line of sight whenever they are outside but I will also be alerted of anything that poses a threat to my family or my pets due to my dog.
I also stated which I don't know if you read that they will only be spending a few hours outside per day- literally three hours added up at about 30 minutes to an hour at a time, and because of Australia's bipolar weather they will most likely be coming inside a lot during those hours that I plan on leaving them out.
So I appreciate your concern but I'm pretty sure I got it covered.