Alcohol Volume by Weight: Great Spirit

NB: Subsection 6(2) of the Distillation Act, subsection 3(2) of the Spirits Act and the preamble to the Schedule to the Excise Tariff Act state, respectively, that the volume of alcohol contained in any liquor or other substance, spirits or goods shall be taken to be the volume that would be the volume of that alcohol if the alcohol were measured at a temperature of 20°C.
Those provisions also specify that the specific gravity of alcohol in relation to water is 0.79067.
The Distillation Act has been repealed and replaced with :- Excise (Alcoholic Strength of Excisable Goods) Determination 2019.
All quantities relating to external deliveries and receipts are required to be corrected to 20°C to correctly assess any duty liability in accordance with the Excise Tariff Act.

What’s happening with the different densities?
ATO / Australian law requires you to use a legal specific gravity for pure ethanol at 20 °C of 0.79067 when calculating excise volume. This is prescribed in the Excise Determination and ATO guidance.
OIML and modern metrology use a slightly different, experimentally-derived value (e.g. OIML / international alco-metric tables list ~0.78924). That value comes from up-to-date densitometry and the international alcoholometry literature (European Pharmacopoeia / OIML tables).

Why they differ

Legal vs scientific standards: the ATO figure is a legal constant chosen for consistency and regulatory stability (so excise calculations are reproducible and not shifting when metrology improves). The OIML value is the modern experimental best estimate. Small magnitude: difference ≈ 0.0014 (about 0.18% relative). It’s small, but material for excise money and duty reporting, so law takes precedence for tax purposes.

Where to find the ATO/legal references

Australian Taxation Office practical guidance page: Measuring excisable alcohol volume and strength (states use 0.79067).
The legislative instrument: Excise (Alcoholic Strength of Excisable Goods) Determination 2019 (text confirming the 0.79067 constant).
ATO guidance and excise law also reference using standard practical alcohol tables (AS2371 historically) for hydrometer conversion.

Practical effect for you

For excise returns / compliance: use 0.79067 and follow the ATO/Determination rules (this is legally required).
For production, lab work, R&D or accurate conversion: use OIML / modern density tables (0.78924 and full tables) — they’re more accurate physically.

How much does a litre of vodka weigh in kg depends on the density of the alcohol in the bottle. A litre of 37% alcohol will weigh 0.953 kg and a litre of 40% alcohol will weigh 0.948 kg.
To calculate rum weight from volume alcohol percentage use an alcohol density table from the volume and alcohol percentage.
The density of ethanol can be calculated from the alcohol percentage.