IMO it depends - on the personalities of both the remandee, and the undercover guy, and the mood of the alleged killer on the day. U/C Guy would need to "strike" when PS appears to be vulnerable/down, or. in a cocky, bragging mood IMO..
Credibility would be crucial. (As Trooper noted - the latter would be kitted out in St Vinnies gear, scruffy...) Ideally he'd ensure that he was seen either smoking, or begging for smokes/asking where he could get some. A couple of home-made tatts on hs hands/wrists wouldn't go astray, and a hefty dose of body odour would be quite helpful.
U/C Guy would need to allude to his contempt for police/remand officers, but in a very understated way - and appear to be a "good bloke" (that indefinable Aussie "thing).
AFAIK all Australian states, and Federal Police have undercover agents if required. I'm not sure about the territories.
Undercover agents aren't limited to remand situations. The work of an undercover agent in South Australia in 2010 was critical in gathering information which led to the arrests of men suspected of committing a horrific murder.
IIRC that U/C Guy happened to meet a chap (who was the farmhand of one of the suspects) at a local pub. They didn't know each other, but got chatting (footy etc, and other things...)
MOO