“I’m Good, Thanks” Can Mean “No, Thank You”

When a server asks, “Would you like some more coffee?”, “I’m good, thanks” usually does not mean “I am a good person”. It means “I have enough at the moment, thank you”. Good describes the speaker’s present situation: they already have what they need, so the phrase works as a relaxed refusal.

A: Do you need a hand?
B: I’m good, thanks.

“I don’t need it” is grammatical but can sound abrupt. “No, thanks” is more explicit, while “I’m good, thanks” is more conversational. It is entirely natural in Australia and is also understood in the United States and Britain; it is not uniquely Australian. After “How are you?”, however, the same words will usually mean that the speaker is well.

https://dictionary.cambridge.org/dictionary/english/i-m-good