Why Is The King On Australian Coins?

By GovMint :

In Australia, the presence of the reigning British monarch on coins reflects the country's status as a constitutional monarchy within the Commonwealth of Nations. As a member of the Commonwealth, Australia honors its historical ties to the British monarchy by featuring the monarch's image on its currency, underscoring the historical connection between Australia and the British Crown. 

Sources Used:

https://www.ramint.gov.au/news-media/news/king-charles-iii-effigy-frequently-asked-questions

https://peo.gov.au/understand-our-parliament/your-questions-on-notice/questions/why-are-we-a-constitutional-monarchy