When I saw this signboard outside a public school, I was honestly baffled at the mental image. I thought it was the Aussies taking a piss out of the mufti (who'd been stirring quite a bit of trouble here). But then, it did sound like some strange kind of event for the Westmead Children's Hospital Bandaged Bear fund raiser. So I looked it up.

A 'mufti day' is when office workers are allowed to wear their home clothes, and in return give money to a good cause. ...
The 'mufti' day dates back to the 1970s, when schools introduced charitable 'Wear what you want days' as a change from sponsored walks and fetes worse than death.
... But 'mufti days' are found in Australia, New Zealand and the Far East. Indeed, the school magazine of St Joseph's in Sarawak, Malaysia, recently published a theory about the event's origins.
"Mufti Day is so named," it said, "because it was inspired by the practice of General Mufti, who found that sagging morale in the British Army was boosted by the policy of allowing the men to wear civilian clothing instead of uniforms on certain days of the year."
~taken from Frantic Semantics