No, the law doesn’t demand that Myki accept 5 cent coins, or that Metcard machines accept notes
The question seems to keep coming up as to whether it’s legal for Myki machines to not accept 5 cent coins; or indeed whether it’s legal for Metcard machines on trams to only accept coins (not notes).
Some people assume that because it’s all legal tender, it must be against the law to demand specific currency, or otherwise limit the payment options (such as only providing a limited amount of change).
As this page from the Reserve Bank says, legal tender doesn’t mean there’s any obligation to accept it:
It is the Reserve Bank of Australia’s understanding that, although Australian currency has legal tender status, it does not necessarily have to be used in transactions and that refusal to accept payment in legal tender banknotes and coins is not unlawful.
So you might not like it, but it’s not illegal.
PS. Some people even claim stuff like this that they don’t like is unconstitutional, and someone should be taken to court. Good luck with that.