Roestbakken is still coming to grips with the late call for her to join the Matildas at the tournament in France, after she was added to the squad as an injury replacement last week.

The teenage defender has stamped herself as a long-term international prospect after relishing in the Matildas' development program and successfully leading the Young Matildas in April.

The Future Matildas program is aimed at giving young players a stepping stone to senior international duties by helping them make a quick transition from the junior system.

"Coming off W-League and then moving to Sydney with the Future Matildas has helped me immensely," Roestbakken said.

"Because we train with a few of the Matildas, it is really helpful being there, being able to train with them and do what they do.

"I am definitely learning a lot and I am getting more experience within international football, and learning what it takes to be a professional athlete."

The 18-year-old was left out of the original 23-player Matildas squad, but an injury to defender Laura Alleway opened the door for Roestbakken to join the team in France.

"I got a call from [team manager] Vito [Basile] at home in Sydney and I just couldn't believe what he was saying," Roestbakken said.

"I got off the phone and called my parents and my dad and my mum were speechless, they couldn't believe it either.

"To get the call up to come to a World Cup is just an unreal feeling and it couldn't have happened at a better time, so I am so honoured and grateful to be standing here today."

Roestbakken is relishing the chance to wear the green and gold of Australia but admits she was left shattered by the suspected foot fracture that has derailed Alleway's World Cup hopes.

"I am absolutely gutted for Laura, she is an amazing player and she does deserve to be here," Roestbakken said.

"These things happen in football and it is unfortunate, and I wish her all the best in her recovery."

Roestbakken didn't play in the Matildas' World Cup opener, which ended in a 2-1 defeat to Italy, a nation that hadn't featured in the major tournament in 20 years.

The tactics used by head coach Ante Milicic garnered a lot of criticism from footballing experts, with Canberra United coach Heather Garriock labeling the performance as "disastrous."

But Roestbakken believes the side will use the shock loss as motivation for the remainder of the campaign.

"The girls are obviously gutted for the first loss, but it is the first game and you have got to deal with these things in football," Roestbakken said.

"I definitely think it will fire them up and they will be ready to go on Thursday."

Australia lost their opening game at the two previous World Cups in 2011 and 2015 and still managed to make the finals.

They will look to get their campaign back on track when they play Brazil in Montpellier at 2am AEST on Friday.