Too many variables at play here. Decisions for a cold climate are not relevant for a temperate one but so many people still believe that more insulation is better - this is not necessarily the case! Insulation decisions for Sydney are similar to those for Perth but still, not the same.
Is the roof covered in solar panels? If so, this will keep the sun off the roof and potentially remove the need for sarking / anticon.
Do you want to go with ventilation and open eaves? If so, you need a lot of whirlybirds and ventilated eaves to pump enough air for it to properly work - a couple of whirlybirds isn't going to cut it (unless the roof is shaded by solar panels of course!). If you go down this path then a conductive / convective heat barrier under the tiles won't do anything. The air flow takes it all away. But you still need a radiant heat barrier (foil) and an air gap to make the radiant heat barrier work (again, unless the roof is covered in solar panels).
In a brick veneer build, then reverse brick veneer becomes the way to go and there must be an air gap between the cladding and the foil then batt insulation in the wall. In conventional brick veneer, you still want the air gap between the bricks and the insulation - you just get no benefit from the thermal mass of the bricks.
In both cases, ceiling insulation that transitions across to the wall insulation (covering the cornice) is critical. If there is a radiant heat barrier under the tiles (or on top of them in the form of solar panels) then you don't need one in the ceiling as well. A high R rating (e.g. R4) in Sydney would be more than enough - I believe you will only get payback on a lower R value than that in Sydney's climate but I (like others) like to be conservative given that it's not easy to get another crack at it. And then of course you have to think about sufficient vapour transmission... which is a good reason not to go too far with the insulation - completely different in a cold climate of course.
All this assumes a conventional roof (a safe assumption for tiles...) - considerations change again if we start talking about skillion etc. And the biggest difference in an Australian temperate climate like Sydney is not found in the insulation anyway - it really is mostly about where the windows and garages etc are. So it really isn't easy to navigate what is effective in certain climates and what isn't.
Unfortunately the computer models that attempt to quantify this stuff aren't yet up to scratch, particularly on that latter point, so everyone in the industry is just focussed on making the model spit out the right result - it's all they know. So you really have to DYOR and arrive at your own conclusions - I recommend to just think about having the right radiant, convective and conductive heat barriers in place for the climate you're in (e.g. foil and air gap or solar panel for radiant, batts for conductive, foil for convective etc) alongside adequate vapour transmission and research from there.
