Not specifically to enforce a lockdown (and to clarify they are assisting police, it's not the ADF doing this by itself), but it's routine for the ADF to assist with disasters.

Surely you remember the ADF helping out with the bushfires not too long ago? There were the big news items like civilians trapped by bushfires being evacuated by the RAN off of Mallacoota beach, but the ADF also provided an incredible amount of assistance with logistics, engineering, and so on... it is deeply unfair to assume the worst of them just because it's enforcing a lockdown.