Car Hire in Australia: need for SLI?

In summary, is it sensible to take out Supplementary Liability
Insurance (SLI) when hiring cars in Victoria, New South Wales, Northern
Territory and Queensland?

I have read past threads about car hire insurance but cannot find a
clear answer. This is partly because I cannot find out
just what is covered by the mandatory insurance.

By way of background:

o we are UK residents
o we shall be hiring cars in Melbourne, Sydney, Ayers Rock and Cairns
o we intend to take out separate insurance against excess, tyres,
windows etc from Questor or the like since the hire companies charges
seem exorbitant (eg AUD33 a day from Thrifty in Ayers Rock).
o the separate insurance providers invariably offer policies with and
without SLI. I know that SLI is highly desirable (in my view essential
unless one is a gambler) in the USA and Canada where there is otherwise
little or no cover for third party liabilities

What I can't find is the position in Australia (in the
States/Territories
mentioned above) as regards damage to third parties and third party
property. The hirers sites generally just say that compulsory third
party insurance imposed by the various State and Territory Registration
Authorities is included in the cost (often with a fee shown for it).
But I can't see what it covers and for how much.

Any advice - if only on where to look - would be much appreciated.

--
R