Ash Gardner has been denied a shot at the fastest ODI half-century after blazing 48 runs off just 18 balls in Australia's win over New Zealand.

On Sunday, Ash Gardner played one of the great World Cup innings, blazing 48 off just 18 balls as Australia crushed New Zealand in Wellington.

But 11 days earlier, she took a routine COVID-19 test, looked down to see a faint line and her tournament flashed before her eyes.

"I won't repeat what I said," Gardner said. "There were a lot of emotions.

"I was trying to blame the air con at the start with a sore throat. When it came back, I was like, 'OK, this is probably not ideal'.

"I had to do a second RAT and those lines came up even clearer.

"I sent a photo to the doctor and basically said, 'I've got some bad news' and he pretty much replied, 'yep that's bad news'.

The prospect of missing a World Cup was a devastating one for Gardner, the reigning Belinda Clark Medallist and key piece of the Australian puzzle.

From the positive test, Gardner went straight into isolation at a Christchurch hotel.

She kept as busy as possible, taking her allowed walks and watching the tournament begin without her.

"I'm not a good watcher of cricket, watching my side play cricket. That first game against England was really hard to watch," she said.

While the mandatory isolation stint was 10 days, or Australia's first two matches, there was no guarantee she wouldn't suffer serious illness, or long-lasting symptoms.

"I was pretty lucky really ... I just had a consistent headache probably for about a week and then from a week onwards to the end of the isolation period, I felt back to normal," she said.

"Early on (in the game) I was getting a little bit puffed running on the boundary, but the main thing out of the game was I just felt like I had a really heavy chest at points, which is quite a weird feeling," she said.

"Other than that I'm feeling pretty good."

Gardner, who re-joined the camp just 24 hours before the 141-run win over New Zealand, certainly didn't show her illness.

The 24-year-old came to the crease with Australia on 5-214 and five overs left.

She struck an open-faced drive for four off her first ball, and didn't let up, ending with four fours and four sixes in a whirlwind innings.

If Gardner didn't run out of overs, she would have had three balls to beat the all-time fastest ODI half-century, struck by West Indies' Deandra Dottin in 2013 off 22 balls against Sri Lanka.

"The platform that the girls set for me to go out there with was really good and I just had that freedom to play the natural game that I play and take the bowlers on," she said.

Gardner wasn't finished, taking the wickets of Amy Satterthwaite and Lea Tahuhu for figures of 2-15.

Captain Meg Lanning joked the performance might change the way she treats her star allrounder.

"Training may be overrated," she said.

"It's great to have her back ... she's come back incredibly well and looking forward to the rest of the tournament."