Is sarking truly an effective insulation

I have a almost four year old house in Canberra, which was built to the insulation requirements for ACT. The house has parking under the roof tiles, which, according to various tables should result in something like R3 insulation! However, I have a weather-station with three remote sensors, one of which is in the roof and another outside (but under cover); I can monitor these remote sensors on my iPhone. The roof sensor results are showing visually no discernible difference between the outside temperature and the roof temperature - there is only the slightest lag when there is a temperature change.
The only time there is any big difference between outside and roof space temper is when it is sunny, in which case the roof space temperature is much higher, in summer it is 15℃ or more above the ambient outside temperature.
From my small scale experience it appears that sarking does not provide anywhere near the insulation level that it supposed to. I am not saying it is not useless, but that it is not that effective. My main concern is that my HVAC is all in the roof space sitting in the cold (freezing) in winter and super heated roof space. The ducting is lucky to have an R1 insulation, the plenum has nothing and neither do the heater and refrigeration unit - so how much is it costing me because of losses into this uninsulated space?
I am therefore wondering what experience is of other people and what solutions are there?

I don’t know about its insulation qualities but I do know that it is almost essential under iron roofs to reduce/eliminate condensation issues in roofs.

Try a thermal coating for the roof much like thermaguard/thermashield or Sky cool ronb

It is quite a technical topic. Different roof materials have very different characteristics. Some are good insulators, conductors, radiators, some display the Silver Tea pot character, some are a good source of heat retention!

Then there is the roof space itself. Does it have insulation only as Sarking, ie, directly in contact with its roofing, or does it also have ceiling insulation, what about roof venting and draught controls. Still air can be an insulator.

50mm of insulation and some tin foil, the typical sarking, is unlikely to provide R3.

A well planned tin roof, would typically have 50mm sarking, to minimise summer heat coming in, a roof vent to allow hot air to escape, and 100mm+ ceiling insulation. The heat within that roof space might seem hot in summer certainly hotter than outside, but the ceiling insulation would stop that penetrating into the house. In winter the ceiling insulation will minimise the heat loss into the roof space.

I thought primary purpose of sarking is to stop water transfer / penetration into buildings.
Insulation is an extra.
Is the R3 , for heating or Cooling?