Because Australia suppressed the virus as much as they could with a very long lockdown. South America didn't, so they started with complete different baselines into their summers.
Only one state in Australia - Victoria - had a long lockdown. People in most other states have been living a fairly normal life for months.
The population of South America is more than 15 times larger.
My comment was based on deaths per capita[0], which in most countries of South America are far higher than Australia and New Zealand.