First, no new laws came into effect - new restrictions under existing law did.
Specifically, the Premier declared a State of Disaster under s23(1) of the Emergency Management Act 1986 which requires it to be broadcast from a broadcasting station in Victoria (the televised press conference did that) and published in the government gazette (see first link). The government gazette is the communication channel for the promulgation of law - once it's in that then you are deemed to know it.
The specific restrictions were communicated in the press conference and the power to impose them comes from s24(2)(d):
(2) In addition to and without in any way limiting the generality of subsection (1), in a state of disaster the Minister may—
(d) control and restrict entry into, movement within and departure from the disaster area or any part of it
Note that this is a different Act than the previous restrictions which were under s200 of the public Health and Wellbeing Act 2008.
Second, ignorance is only an excuse when the law says it is an excuse. Neither of the above laws do that. Most laws don't do that.
An example of one that does (that I know from personal experience) is s58 of the new-south-wales Road Transport (Vehicle Registration) Regulation 2017 which says:
For the purposes of section 68(2)(b) of the Act, the use of an unregistered registrable vehicle on a road or on a road related area is permitted if the person using the vehicle--
(a) was not the responsible person for the vehicle at the relevant time, and
(b) did not know, and could not reasonably have known, that the vehicle was unregistered at the relevant time.
However, even here, the ignorance plea is in relation to the facts of the situation, not the applicable law.
My son drove my company car while unregistered and pled this defence. The regulator denied that he had a reasonable excuse because he should have checked the registration status using the NSW Government App before driving it. That is, my son had a duty to make all reasonable efforts to establish the registration status of the vehicle. However, they used their discretion and waived the fine.
Which brings me to ...
Third, officers, prosecutors and judges have discretion in how they apply the law. While ignorance is not an excuse, it is a factor that they can consider in deciding if you should be penalised and how much that penalty should be.