McGregor, sweltering in the Spiegeltent in neck-to-ankle sequins, can do just about anything with her voice – she has range, power, depth, emotion and a whistle register that would make Mariah blush.

And so she does, turning Britney Spears’ “Oops! I Did it Again”, into a New Orleans stride, effective enough, she says, to fool legendary Australian jazz musician James Morrison.

Backed by an impeccable band of Sam Keevers on keyboard, Lyndon Grey on bass and Hugh Harvey on drums, McGregor takes her adolescent love of pop and melds it with her study of the three great women of vocal jazz – Ella Fitzgerald, Billie Holiday and Nina Simone – to create a set of enormous fun, and surprising depth.

With great humour and ease, she had the audience at once laughing, and then gasping – she has amazing range.

INXS’s “Never Tear Us Apart”(with someone looking a lot like the Port Adelaide coach sitting in the second row), becomes a searing torch song, the Fine Young Cannibals’ “Johnny We’re Sorry” is crafted into a poignant lament (with shades of Shirley Bassey), and the classic of the American songbook, “Cry Me a River”, is seamlessly melded with the Justin Timberlake number of the same name.

And then things get really crazy, and even more impressive.

A medley by 1990s Brit favourites The Prodigy (yes, including “Smack My Bitch Up”), is morphed into a boogie-woogie blues number, and the Foo Fighters’ “Best of You” becomes a heart-breaking, soaring break-up standard.

Picking up the bizarre Suzuki Omnichord (a 1980s instrument that sounds like a cheeseball electronic organ) she seeks to reclaim the “worst cover version” every produced – Celine Dion and Anastacia singing AC-DC’s “You Shook Me All Night Long” (and it is truly awful), before launching into another medley, this time of 1980’s “lady” hip-hop featuring Salt-N-Pepa’s “Push It”.

McGregor does get you laughing, but she can also make you catch your breath.

Her final number is an aching cover of Radiohead’s “Creep”, in which she starts like Barbra Streisand and ends like nothing but herself – powerful, empathetic, and a little bit miraculous.

Five stars

Ali McGregor played for one night only in the Spiegeltent at The Garden of Unearthly Delights.