According to the regulation (ADR 81/02), the weighted fuel consumption (1.9l/100kms for PHEV) is calculated from two separate tests which derive Condition A fuel consumption and Condition B fuel consumption where:
Condition A: Test shall be carried out with a fully charged electrical energy/power storage device
Condition B: Test shall be carried out with an electrical energy/power storage device in minimum state of charge (maximum discharge of capacity)
and then the Weighted Fuel Consumption value is calculated by following formula
C = (De·C1 + Dav·C2)/(De + Dav)
C = fuel consumption in l/100 km.
C1 = fuel consumption in l/100 km with a fully charged electrical energy/power storage device.
C2 = fuel consumption in l/100 km with an electrical energy/power storage device in minimum state of charge (maximum discharge of capacity).
De = vehicle's electric range, according to the procedure described in Annex 9, where the manufacturer must provide the means for performing the measurement with the vehicle running in pure electric operating state.
Dav = 25 km (assumed average distance between two battery recharges
On a PHEV, other relevant data reported under ADR 81/02 and shown on the Fuel Consumption Label (and the Green Vehicle Guide www.greenvehicleguide.gov.au) is Energy Consumption in Whr/km. When you combine the Energy Consumption and the Fuel Consumption, you'll get an idea about the actual efficiency of the vehicle.
Depending on how much driving time is spent in EV mode compared to Hybrid Driving Modes, a customer may well achieve significantly better or worse than 1.9l/100kms.
About the Testing Procedure
All vehicles are tested to the same test procedure (drive cycle) under carefully controlled conditions in specialised vehicle emission laboratories. The test methods used for determining exhaust emissions and fuel consumption are specified in the ADRs. The same drive cycle is used for determining air pollutant emissions, CO2 emissions and fuel consumption. The ADRs adopt the test methods from the United Nations ECE Regulations (ECE R83 and ECE R101). As the results displayed in the GVG are based on a standardised drive cycle, different vehicle models can be compared with confidence. However, no test can simulate all possible combinations of conditions that may be experienced on the road. Real world emissions and fuel consumption may vary from the results provided in the Green Vehicle Guide, depending upon a number of factors including driving and road conditions, driver behaviour and the condition of the vehicle.
More information on this can be found on: http://www.greenvehicleguide.gov.au/GVGPublicUI/Information.aspx?type=RatingsAndMeasurements