Understrength Australia will retain the women's Ashes unless England can score 199 runs in the first Twenty20 International of the multi-format series at the SCG.

Veteran opener Beth Mooney (75 runs from 50 balls) led the charge for the hosts, who were rocked by injuries to captain Alyssa Healy (foot) and star allrounder Ash Gardner (calf) before being sent in to bat on Monday.

After playing the steady hand alongside debutant Georgia Voll (21) early on, Mooney cut loose after drinks for a 23rd T20I half-century, and ultimately her third-highest T20I score.

England will need to pull off the equal second-largest run chase in women's T20I history to keep their hopes of winning the Ashes alive, with Australia finishing their 20 overs at 7-198.

Having won all three One Day Internationals, Australia can't be chased down on points aggregate if they win any of the three T20Is, or the one-off Test match.

Mooney rode her luck after being given lives on 16 and 23 runs, first dropped by wicketkeeper Amy Jones.

She then breathed a sigh of relief as her shot high to cover was inexplicably allowed to bounce by Nat Sciver-Brunt and Charlie Dean, the pair miscommunicating just outside the circle.

The sloppy fielding came after Lauren Bell dropped a regulation chance at fine leg that could have dismissed Voll on 13, but England tightened their act up in the field after drinks to get back in the game.

Captain Heather Knight ran Phoebe Litchfield (25) out with a direct hit from 20 metres away after the young gun helped Australia to 1-90 after 10 overs with some dangerous switch-hitting.

Jones stumped Ellyse Perry (7) by the barest of margins, and England would've felt right in the game at 4-121 when fellow allrounder Annabel Sutherland (3) holed out to midwicket.

But Mooney clicked into gear alongside swashbuckling stand-in captain Tahlia McGrath (26 off 9 balls)

Bell was a favourite victim, with Mooney slapping the quick for three fours - all behind the wicket - in the space of four balls in the 16th over.

She drilled Sarah Glenn's full toss straight down the ground for four in another highlight of her blistering innings.

Mooney was eventually stumped by Jones in the 18th over off Freya Kemp's bowling, walking back to the pavilion as the umpire reviewed the play.

Maia Bouchier sent a scare through the England camp after appearing to hurt her left shoulder diving to save a boundary in the 15th over.

Play stopped for staff to attend to the opener, who managed to play on, and will be relied on for a big score as England chase the formidable total.