Hi Macoto,
In this first instance and therefore forgetting who would live there. If you're based in Australia, how would you plan on purchasing the property? Do you have friends / family in the UK you could trust to view on your behalf or would you buy "blind"? (By "blind" I mean purely using photos to make your decision, ie, Rightmove, Zoopla, etc.)
Secondly, I expect at some point every landlord will have some form of horror story, I have 3 BTL's at this moment in time, in 1 house the same lady has lived there with her son for 8 years and we've had no problems.
In another house, my first tenant was fine until 1 weekend when he went away and let his 21 year old son have a surprise bday party for his 16 year old sister, a window was smashed, an original sash window with vintage sash cord had the sash slashed and an external tiled concrete window ledge had a sledgehammer taken to it, needless to say they didn't stay much longer and he didn't have much left of his deposit, the following 2 tenants over the last 3 years have all been fine.
In my final property, a 1 bed flat, the first 3 months were all ok but then the tenant lost his job and couldn't afford to pay the rent so had to terminate his agreement by mutual consent.
Finally, the difference between letting to a stranger through an agent, or privantely, than letting to family is that at the end of the day it's still your house and you will still have to maintain it. Family may be more reliable in keeping the place well looked after but that may be down to their age and mentality, after all, I expect an aunt or uncle in their 30's or 40's will look after it far batter than 1 or 2 cousins in their early 20's.
Personally, if this is to be your first BTL venture then I'd rent through an agent and have them find your tenant and manage the property. Make it clear from the outset you'll be a fully "hands-off" landlord, as in you won't get involved with the upkeep or maintenance, etc, because you're not local and let them do everything for you. Most agents generally charge an initial fee for their services for the inventory, tenant search, credit / reference checks, etc and will then charge a 10% or so monthly fee. Depending on your deposit, with the right property and mortgage, you could still make a healthy profit after all fees.
Good luck with it and keep us updated with how you get on. There's plenty of other threads here that will give advice on areas to look at, BTL tips, etc so worth having a full read through as and when you can.