The scary number, whose name I forget, is the reinfection time. The time from when a person is infected until they start infecting others. That number for "ordinary" Covid is something like 4-5 days. This new British variant that is causing us grief has a period of 48 to 72 hours.
That means that the victim is infectious before or at the same time as they develop symptoms.
The Five day lockdown is to enable the contact tracers to keep up with the virus. There are already about a thousand contacts isolating in Victoria.
What you should be concerned about is anyone who has traveled through terminal 4.