If you want to do it, I reckon it would work better as a response to the haka. As soon as the haka finishes have a crack, and have a crack loud. That is when the Aussies normally go back to a huddle to compose themselves, but imagine the lift with a spontaneously sung supporter song.
Most likely to work is waltzing matilda, as almost all have done it before at the rugby.
IMO this is the only way it will work with most of the people (who are willing) on the same page. With a flyer;
Aussie response to the Haka
let the wallabies hear your support
Haka, respectful golf clap,
then Chorus (to get everyone together), Verse, Chorus (rousing finale)
Waltzing Matilda, Waltzing Matilda,
You'll come a Waltzing Matilda with me,
And he sang as he watched and waited till his billy boil
You'll come a Waltzing Matilda with me.
Once a jolly swagman camped by a billabong,
Under the shade of a Coolibah tree,
And he sang as he watched and waited till his billy boil,
You'll come a Waltzing Matilda with me.
Waltzing Matilda, Waltzing Matilda,
You'll come a Waltzing Matilda with me,
And he sang as he watched and waited till his billy boil
You'll come a Waltzing Matilda with me.