Do I have to apply for a visa when I return to Australia after I get an Australian green card?

When you go back to Australia with an Australian green card, you don't need a visa.

Rights and obligations of a green card:

A green card is essentially a valid visa, which generally only allows the holder to enter and leave Australia within five years. After the expiration, it must be renewed. The renewal requires that the holder must have lived in Australia for two years in the past five years. Once you get Australian citizenship, whether you live in Australia or not, you will remain Australian citizenship for life and be protected by Australian overseas consulates.

green card holders who have been sentenced to more than 12 months in Australia will have their green cards revoked. Australian citizenship cannot be revoked.

If one of the parents is an Australian green card holder, and if the child is born in Australia, then the child is born an Australian citizen. If the child was born in China, then the child is a citizen of China, and of course he can apply for an Australian green card in the future. If one of the parents is an Australian citizen, no matter where the child was born (such as China), the child is born an Australian citizen.