Foreign crews (including F/As) are not required to display ASICs in secure areas, an airline ID and uniform are sufficient. The lawful reason for entry requirement also applies.
From the Aviation Transport Security Regulations:
3.05 Crew of foreign and state aircraft etc
Despite regulation 3.03, the following people need not display
an ASIC in a secure area:
(a) a person who is a member of the crew of a foreign aircraft
(other than a state aircraft) that is engaged in a regular
public transport operation or a charter operation and who:
(i) is in the uniform of the aircraft operator; and
(ii) displays appropriate identification issued or
authorised by the aircraft operator:
But these F/As are on AUSTRALIAN registered aircraft not foreign registered so 3.05(a) would not apply to them.
If they don't require ASICs because they are foreign crews then they have no business working in Australia on domestic sectors operated by an Australian airline. Let them obtain work permits first. Jetstar can justify to immigration why they cannot find locals to act as cabin crew within Australia.
Suppose Air Niugini DHC8 crews came to Cairns and started operating for SKYTRANS flying "tag" sectors to Townsville.