family sharing across countries

Only if their account is set to Australia.

https://www.apple.com/legal/internet-services/itunes/us/terms.html

"All Family members must share the same Home Country."

Share purchases with others in your Family Sharing group

https://support.apple.com/guide/mac-help/share-purchases-with-your-family-mchlc2b37692/mac

"To share purchases, family members must be in the same iTunes Store country or region."

Even then there may be issues:

The section below is copied from the USA Apple Media Services Terms and Conditions document but I suspect they are very similar for all versions of this document --> https://www.apple.com/legal/internet-services/itunes/us/terms.html - Last Updated: September 20, 2021

"Certain Services and Content available to you in your Home Country may not be available to you when traveling outside of your Home Country."

In other words, somebody who has their country set to Australia but is outside of Australia may find certain features do not work.

The devices are connected on the same network ( the network are not shared country to another country ) , you'r internet service provider can't permit , so activating of family sharing accounts will not be possible this way a new family member can't be added .

Read Set up Family Sharing - Apple Support

Join a Family Sharing group in iCloud - Apple Support