Behind Australia's 4-0 demolition of Canada at the Women's World Cup was an order from injured captain Sam Kerr to win the match without her so she could buy time to recover from her calf strain, coach Tony Gustavsson said.

Australia reached the last 16 on Monday with Kerr cheering from the sidelines as a Hayley Raso double put Australia in command before a Mary Fowler strike and a Steph Catley penalty completed the rout at Melbourne Rectangular Stadium.

There were concerns over Kerr's fitness when the striker took no part in the Matildas' pre-match warmup.

But with her teammates stepping up in her absence to knock out the Olympic champions, there was no need to risk playing her.

"Part of the reason why I also think that it was so united and (we) performed the way we did is (because) one of the words Sam said to the team," Gustavsson told reporters.

"It is: 'Make sure you win without me so that I can get another week to train and recover and get healthy'.

"And the team said 'Yes, we will. You sit on the bench tonight.'

"You could almost see that commitment from the players (saying) 'Let's do it, you know, so that Sam gets another week of training'."

Gustavsson said Kerr, who missed the 1-0 win over Ireland and 3-2 defeat by Nigeria, had been cleared to play at least some part of the Canada match by medical staff.

However, he said he had an inkling he would not need her.

"It felt like before the game the players were so convinced they could do it without her," he added. "We kind of just said 'Let's just focus on that first'."

Catley led the side in Kerr's absence with aplomb, setting up Raso's first goal and stepping up to nail the spot kick in stoppage time when Katrina Gorry was fouled.

The win put Australia top of group B and eased pressure on Gustavsson after the 3-2 shock by Nigeria where his tactics and substitutions were met with criticism by home media.

Australia will play their last 16 clash in Sydney in a week, most likely against Denmark.

The Swede said the Canada win wasn't about proving people wrong but rewarding fans for their faith.

"I think it represents so much more – what the Matildas stand for and what they want to do in terms of inspiring people," he added.

"It feels good to celebrate with all of you with a night like this but I'm going to be boring and say, 'Let's just do it tonight' because tomorrow we start the prep again."