What is the legal basis of COVID-19 restrictions?

A state of emergency was declared in Victoria from 16 March 2020 until midnight on 13 April in order to combat the COVID-19 threat.

This enlivens a number of powers of the Chief Health Officer, including the ability to make directions under the Public Health and Wellbeing Act 2008 (Vic). 

Although public statements are being made by the Prime Minister, most legislative implementation of those positions are being implemented at the state level.  The full text of each of the directions are available on the Victorian Department of Health and Human Services website: https://www.dhhs.vic.gov.au/state-emergency