Ask An Australian

The term 'cooked' has been around for ages used to describe someone who is crazy in a manner that is consistent with persons suffering from long term alcohol/drug/substance abuse i.e "their brain is cooked"

I think the more recent rise of meth as the country's most pre-eminent substance abuse social problem has helped spawn the term 'cooker' as this has heavy connotations with how meth is consumed and manufactured.

Now with the even more recent pandemic outing a lot of controversial opinions, the term cooker is now concreted into the vernacular as an insult for persons who refuse to grasp basic concepts regarding public health and espouse outlandish paranoid theories in response. Things that most would suppose a person whose brain has been fried by meth would do. Though in my opinion it's a bit harsh on the meth users.