I was surprised to learn that Leo McKern was Australian. Watching him in Man for all Seasons, Rumpole and Reilly, Ace of Spies I assumed his was just another kind of English accent to go with Henry Higgins and Eliza Doolittle. But the English instantly recognize his origin? Or did he do a good job of masking it? How do we Americans tell?
Leo McKern’s normal speaking voice remained distinctively Australian, although reflecting a long life in the UK and using his voice in acting. His Rumpole voice sounded to me (an Australian viewer) as a generalised and slightly rough-throated version of RP English, although I knew he was an Aussie.
Australian accent is commonly mimicked by nasal modification - pinch the nose or try to talk without moving the upper lip.
Helpful primer on distinguishing our accent from our near neighbours.
Isn’t the Aussie version of a long “A” closer to the RP version of a long “I”, á la the above-mentioned Eliza Doolittle pre-transformation? (“No one taught her ‘take’ instead of ‘tike.’”)
Mostly the slang.
To me the Aussie accent is pretty much instantly recognizable and not at all like most British accents. Many British and Aussie actors can imitate Americans really well, but the reverse is not true. That said, my English DIL, who is a born and raised Mancunian, can spot an English actor posing as an American almost 100% of the time.
What is a British accent? The UK has so many regional accents within that it would be hard to distinguish them all. Talking to someone from Ireland, or Scotland, or Wales, or Liverpool, is not the same. You may not even understand them. And the area is not really very large. In the US, a much larger country, you may find some small regional accents but nothing like the UK. Except for New Jersey. (that is a joke)
Australia is a more homogeneous accent. No Sidney vs Outback accents and it is a large country too. Maybe the locals can tell.
Australian actors in US movies and TV blend in better than a British actor.
In my experience, an upper-class/educated register is significantly different (and closer to a British accent) from your stereotypical Paul Hogan shrimp-on-the-barbie accent.
Note that Australians do not use the word “shrimp” for the food but call it a “prawn”.
People like Sir Robert Menzies or Air Vice Marshal Donald Bennett scarcely sound Australian at all.
I think of Aussie vowels as ‘rounder’ and held longer.
Yes, how ‘razor blades’ becomes rise up lights.
McKern did a lot of Shakespeare in England in the 1950s, at the Old Vic and at Stratford on Avon, which at that time must have meant masking his Australian accent and using RP / BBC English.
Generally three main Australian accents are recognised
- ‘broad’ (bushie, bogan, long drawn out vowels, lots of slanging) - Steve Urwin and stereotypical advertising Aussies
- cultivated (English tones, softer vowels, breathier) - our Kylie, Cate Blanchett
- general (normal, mid-range) - Margot Robbie, Chris Hemsworth and other typical Australians
We also have woglish - heavily, deliberately accented English used by first generation Australians - lots of variants depending on where their parents came from. This can be switched on and off at will.
Weird posho accent mainly found in Adelaide and Brisbane, which is very English sounding.
Apart from the last, these accents are class and context based and not at all regional, and can be found co-existing on the same bus anywhere.
A lot of the time they’re got a dead-on generic American accent, which is kind of a giveaway, because in real life, nobody’s quite as non-regional American as the Brits manage. Think Hugh Laurie in House MD. Great accent, but there was something of the uncanny valley about it. Same for Charlie Hunnam in Pacific Rim- generic American, but nowhere in particular, and something was just a hair off.
As far as telling Aussies from Brits on TV and in movies, it’s just something you can hear. It’s hard for me to describe, but I don’t have a lot of trouble.
There’s a slight accent to WA - which to me sounds like a hint of South African - probably because so many white South Africans emigrated to Perth.
Do any of the indigenous peoples of Australia have a distinctive accent, or would they fall into one of those basic categories?
Hell yes. It’s actually an accent that is very pleasing to the ear (to my ear anyway).
However the strength of the accent varies substantantially on a continuum depending on the extent to which their background is living within the general Australian melting pot, or living in places dominated by indigenous. City vs country to some extent.
Listen to this fellow from 0:40 - I could listen to him talk all day.
The Aboriginal people I know are mainly urban, southeast Australian and use the full range of Australian accents. Also, when the audience is predominantly Aboriginal people can go into a stronger lingo with particular accents, words from various languages that remain in use, esp when its more social and bantering.
Indigenous people whose second or third language is English tend to have a particular pronunciation, as not all English sounds are present in Aboriginal languages.
There is a stereotypical Aboriginal accent that get wheeled out for e.g. historical dramas. Its a bit like every native in cowboy films speaking Standard Injun.
IME Australian actors are extremely good at mimicking English accents, to the point we can’t spot them. Perhaps they’re just more exposed to them than American actors.
There is no ‘British’ accent! 
The nearest thing is an ‘English’ accent called ‘Received Pronunciation’. 
Received Pronunciation - Wikipedia
Within the UK Scotland, Wales and Northern Ireland all have their own language (plus regional accents.)
As for ‘English’ accents, there are many e.g. Cockney (London), Scouse (Liverpool), West Country, Geordie (Newcastle) and Birmingham.
I once helped two English speakers with some ‘translation’ - one came from Scotland and the other from Newcastle.