Sydney FC defender Ally Green has issued a warning to rival A-League Women sides who hope to get an easier ride with the Sky Blues' Cortnee Vine and Remy Siemsen on international duty.

Vine and Siemsen were called up to the Matildas' Asian Cup squad last week after leading the Sky Blues line in devastating fashion to start the season.

The duo has scored four goals each and while Siemsen is locked in the 23-player squad, Vine must battle it out for one of two remaining spots.

Sydney face Canberra on Saturday at Kogarah Oval without both players for the first time this season, and Green said the onus now fell on others to step up in their absence.

"It's a pretty important time of the season but we've put ourselves in a good position," Green told AAP.

"Now it's mid-season and we are hoping to keep scoring goals.

"Obviously a lot of that so far has come through Cortnee and Remy who are now away with the national team.

"They are unstoppable at times, but we've got substitutes sitting on our bench who could start for most teams.

"That's been Maria Rojas and someone like her coming into the side will be raring to go.

"She can provide a different form of attack and she can be dangerous up top."

While Sydney's goalscoring has been exemplary, their work at the back has largely been unheralded.

The Sky Blues are undefeated to start the season under Ante Juric's guidance and the sole goal they conceded was in their round one win over Newcastle.

Green has been integral to that success and while she could be considered unlucky to miss out on a Matildas berth herself, the 23-year-old is hoping to keep Sydney's stonewall defence intact.

"Our defence has been really solid. It's something we're pretty proud of," she added.

"I think we can be a little more clinical with our passing.

"Whoever has been in our backline has done the job and for us. As a squad we really want to keep that going."