Ask HN: Are there many Australians well known for their open source work?

Yes, for example https://ozlabs.org/ (I've worked with several of these people, and one prominent example is Andrew Tridgell, original developer of Samba and rsync, among other things).

SSLeay, which was forked and became OpenSSL, was developed by Tim Hudson (AU) and Eric Young (NZ).

Also, he's not precisely (primarily?) an open source developer, but Julian Assange is Australian.

It's a niche area, but Simon Newton of ola (Open Lighting Architecture) - https://github.com/nomis52