...and nobody says "oz" when they mean to say "ounce". These are literal liberties, which fit either way, I think.
Voicing s as /z/ is widely variational. For example, I voice the s in "us", and "Muslim", while others don't. While I don't voice the s in Australia (/ɒ.ˈstreɪ̯.li.jə/ ~ /ə.ˈstreɪ̯.li.jə/), I do in Oslo (pronouncing it /ˈɒz.loʊ̯/).
Pronouncing 'Oslo' with a z sound doesn't mean 'Australia' is pronounced with a z sound, you're comparing the... you know, I don't care.
They do say it with a /z/ sound. That's why it gets spelled Oz.
Compare Aussie, which is also pronounced with a /z/.
I've never heard anyone pronounce 'Australia' as 'Auztralia'. 'Aussie' as 'ozzie' yes, but never Australia.