Australia captain and forward Sam Kerr was ruled out of two games of the Women's World Cup, including the opener against Ireland on Thursday, after picking up a calf injury in training a day earlier.

Kerr's loss is a big blow for Tony Gustavsson's 10th-ranked Matildas, who are considered one of the favourites to win their home tournament. The 29-year-old Chelsea striker is the all time leading goalscorer for her country with 63 goals.

"I wanted to share this with everyone so there is no distraction from us doing what we came here to achieve," Kerr posted on Instagram barely an hour before kick-off.

"Of course I would have loved to have been out there tonight but I can't wait to be a part of this amazing journey which starts now."

The team said she would re-assessed by the medical team after Australia's second group stage match on July 27 against Nigeria.

Australia kicked off the tournament on a wave of momentum having won nine of their previous 10 games, including a 1-0 win over France in a friendly last Friday and a 2-0 win over England in April that ended the Lionesses' 30-game unbeaten streak.

There was some injury speculation around Kerr after the France game when she was spotted with an ice pack on her hamstring after she was taken off in the 62nd minute.

This is the fourth World Cup for Kerr, who spoke about the pride of playing on the global stage in front of a home crowd.

"To be at home and get the feel for it, walk down the streets and see people that are excited or flying around the country to watch us, it's nice to be able to feel the love," she said on Wednesday.

"But also we're really proud to bring this World Cup home and let Australia experience what we do all around the world."

Thursday's match against Ireland, who are making their first appearance at a major tournament, was expected to be played in front of a capacity crowd at Stadium Australia. Kerr was in the dugout and linked arms with the team's substitutes during the singing of the national anthem.

Fans entering the stadium were crestfallen to hear the news.

"Oh, that's not good, that's very bad, that's awful, she's the best player we have so that's bad for us," said supporter Matt Starr.

"Very sad, we're very sad, we came to see Sam Kerr but still Australia to win," said fan Sue Moar.

Lisa Schibezi called the news "devastating" and hopes Australia make it through the group stage so Kerr has the opportunity to play in Sydney.

"Because this is our only chance to see her in Sydney so if they don't make it through, we don't get to see her play in Sydney, so just really really hope she's okay and we get to see her again," she said.