Its not a pleasant situation at all.  For a State to bring in the Army against its own people is a very big step.  I don't even know whether the law (as it stands in 'peace time') allows for it.  Basically the offenders need to be arrested, processed and brought before the court.  This is not something the Army can do against civilians.

Basically Australian law is loosely based on English Law, hence in Victoria section 462a of the Crimes Act covers your question:

Use of force to prevent the commission of an indictable offence

462A. Use of force to prevent the commission of an indictable offence

A person may use such force not disproportionate to the objective as he
believes on reasonable grounds to be necessary to prevent the commission,
continuance or completion of an indictable offence or to effect or assist in
effecting the lawful arrest of a person committing or suspected of committing
any offence.

Source :http://www.austlii.edu.au/au/legis/vic/consol_act/ca195882/s462a.html

You'll note a couple of things.  There is NO reference to a Police Officer in the above.  Its ANY PERSON.
Secondly is that the force is not disproportionate to the objective.

Without being in the UK at the moment, its hard to comment however from the media footage the vast majority of person(s) rioting appear to be teenagers to mid 20's.  The parents of the teens need to drag their children home and lock them in until things settle down.