Well, considering the WA CHO testified in court that their health system can
successfully manage 500 new cases per day the answer is obviously not "1". That would utilise 650 hospital and 124 ICU beds,
but would remain within the capability of the WA health system. That being the case, I would say anything more than 500 new cases per day, based on a population of 2.66 million should be the trigger to require a lockdown, because anything
less than 500 can be managed by the (existing) health system. Your reference is
Palmer v Western Australia (No 4) [2020] FCA 1221 [86].
If you want to extrapolate that to other states, on the assumption all health systems have the same capacity, that is 1,200 cases a day for Mexico and 1,500 for NSW. And that's
per day remember....