Originally Answered:

Why do British/ Irish/ Australians when singing have the same American accent as American singers?

Brian Collins and Robert Charles Lee, I disagree. They do too. And I do have a tin ear, but I’m not the only one to think so:

• Non-American pop singers (e.g. New Zealanders) tend to subconsciously adopt an American accent in singing. Is this observed in languages other than English as well?
• Why do many British singers sound American when they sing?

And I reckon it’s a genre effect. Pop/rock is American, so non-Americans sing like the Americans they’re emulating. John Williamson (singer), by contrast, does Australian Country with some overt Australian nationalism; and his vowels are, well, True Blue Aussie:

TISM were suffused to the gills with in-joke references to Australian culture. I got into them when I left Australia, out of nostalgia. I agree you wouldn’t mistake them for Americans, but those are not Australian vowels either, and there’s more rhotic action there than there should be. Because they were doing genre.

The two alt-rock bohemian self conscious hippie drongo skip bastard Aussie bands I loathed growing up were Frente! (with exclamation mark!) and Ratcat. The reason I loathed them, apart from being alt-rock bohemian self conscious hippie drongo skip bastards, was that they were among the first Australian acts to make a point of singing pop with an Australian accent:

Sod off, alt-rock bohemian self conscious hippie drongo skip bastards. Give me the grunge stylings of Silverchair any day.

With a mumbly Seattle accent: just right for a bunch of teenagers from Newcastle NSW.