There are two methods used, the Average Recurrence Interval (ARI) and the Intensity Frequency Duration (IFD).
The ARI is more commonly used because the maps are in AS 3500.3 Stormwater Drainage and easy to see.
The IFD takes the rainfall intensities from thousands of weather stations and is more area specific but it takes a little longer to access.
Forum contributor Bashworth has an excellent blog full of great accurate information and the link below takes you to a page that explains the IFD.
Your region's ARI will be either 130mm/hr or 140mm/hr which is based on an average rain intensity per minute over a 5 minute duration. Eaves gutter compliance uses the areas 1:20 rain intensity figure.
You have 100mm x 50mm rectangular downpipes and what looks to be a standard 23 degree roof pitch.
The 6 star regulations means that you will have two downpipes diverted to the tank to comply with the regulation that states that the water tank must be supplied from a minimum 50 sq m roof catchment area. The roof catchment area is greater that the plan area because it is factored for wind driven rain according to the slope. If you don't have two downpipes diverted to the tank, the installation does not meet the 6 star compliance.
If two downpipes are diverted to the tank, then one downpipe will almost certainly be a wet system that runs underground to the tank and then up a vertical riser to the tank's meshed top inlet.
A wet system needs to have a flush valve fitted at the lowest point but this is not always possible.
Most wet systems are sub standard yet compliant.
Other things to look for.....
Leaf diverters fitted to wet system downpipes are not mandatory in Victoria but they should be because they not only prevent mosquitoes from breeding, they also prevent large organic debris and other matter from entering the wet system which provides nutrients to bacteria that can turn the water anaerobic. Bacterial die off also melds with built up sediment and forms a glutenous muck that in time will restrict and even block the flow of water. The best leaf diverter currently available is the ICON Leaf and Debris Controller.
If a first flush diverter is fitted to the vertical riser, it is useless because the diverter will only fill with the settled water in the riser...the first flush will still be in the downpipe at the other end of the wet system.
If you have a submersible pump, make sure that it isn't sitting on the tank's floor vaccuming the sediment layer whenever in use.
If you have a slimline tank, make sure that the wet system infeed pipe isn't directly above a submersible pump. A plunging water jet generating entrained air around a submersible pump's intake can cause all sorts of problems.
The overflow pipe's mesh can also gum up in time, it needs to be accessible by removing the tank's top mesh inlet for maintenance.
If the pump is external, make sure that the tank's outlet valve is not fitted to the bottom of the tank.
Also make sure that they haven't fitted a 90 degree elbow on the suction hose directly to the pump's suction side. A pump's suction hose should respect a straight connection that is 8 internal diameters long.
If the pump causes water hammer, they have specked a pump that is too powerful. This is very common unfortunately.