Why Does ‘Cheers’ Often Mean More Than a Toast in Australia?

In everyday Australian English, cheers very often means “thanks”. If someone holds a door, hands you something or does you a small favour, “Cheers” can sound lighter and more casual than “Thank you very much”. It can also work as an informal goodbye.

For example: “Here’s your coffee.” “Cheers.” There is no toast involved. The tone is friendly and brief, which suits familiar, collegial and everyday service situations. For formal correspondence or serious gratitude, a more explicit thank-you is usually better.

https://dictionary.cambridge.org/dictionary/english/cheers