Mailing Banknotes Overseas

Probably illegal,
if collector notes, that have current legal tender status.
I wouldn't let that worry me too much.

Just be cagey, so that what you write on the Customs Declaration sticker, is such that the package passes X-ray examination, and does not raise Customs Officer curiousity.

Include some copies of relevant documentation in the package to establish your honest bona fides, should the package be opened by Customs officers.

Pack the notes in a Mylar sleeve, between two crossed corrugated cardboard sheets cut to size, and place in a bubble pack postal envelope.

I would imagine that lots of collector notes are moved through the postal systems of most countries, especially The U.S., Canada, The U.K., Europe as well as Australia.