Prince William and his wife are understood to be waiting for the formal invitation from Australian prime minister Scott Morrison.
This represents the first engagement of 2020 for Duchess Kate, who celebrated her birthday on Thursday last week.
Duchess Kate, 37, was pictured, together with Prince George, 6, and Princess Charlotte, 4, by a supermarket customer at the checkout.
The Every Mind Matters website was launched officially yesterday in a short film voiced by the dukes and duchesses of Cambridge and Sussex.
The 'Fab Four' feature alongside one another in a three-minute video that also stars actress Gillian Anderson and presenter Davina McCall.
Princess Charlotte, 5, will be starting school next week on Thursday 5 September, at Thomas’s Battersea.