How Australian NGA Factors Are Used in Emissions Calculations

In Australian greenhouse gas accounting, the National Greenhouse Accounts (NGA) Factors document is the single authoritative source for emission intensities. Published annually by the Department of Climate Change, Energy, the Environment and Water (DCCEEW), it provides the conversion factors that turn physical activity data — litres of diesel, kilowatt-hours of electricity, cubic metres of gas — into CO₂-equivalent tonnes.

Understanding how NGA Factors work, and why the version you use matters, is foundational to producing credible and defensible emissions calculations.

What the NGA Factors document contains

The NGA Factors publication is a comprehensive technical reference covering:

Electricity grid emission factors — separate factors for each state and territory grid (NEM states, WA, NT), updated to reflect changes in the generation mix. These are used for Scope 2 location-based calculations.

Fuel combustion factors — emission intensities for every common fuel type used in Australia: diesel, petrol, LPG, natural gas, aviation turbine fuel, coal (various grades), fuel oil, and more. Each factor expresses the mass of CO₂, CH₄ (methane), and N₂O (nitrous oxide) produced per unit of fuel, combined into a CO₂-equivalent intensity.

Refrigerant and fugitive emission factors — global warming potential (GWP) values for common refrigerants and industrial gases.

Waste and wastewater factors — for organisations with significant waste streams.

The global warming potentials (GWPs) used to convert non-CO₂ gases to CO₂-equivalent. The NGA Factors currently use the IPCC AR5 100-year GWP values (e.g., methane = 28 CO₂e, nitrous oxide = 265 CO₂e).

How the calculation works in practice

The basic emissions calculation is:

Activity data × Emission factor = Emissions (CO₂e)

For diesel combustion:

• Activity data: 10,000 litres of diesel consumed
• NGA factor (diesel, stationary combustion): ~2.68 kg CO₂e per litre
• Emissions: 10,000 × 2.68 = 26,800 kg CO₂e = 26.8 tonnes CO₂e

For purchased electricity in Victoria:

• Activity data: 500,000 kWh consumed
• NGA grid factor (Victoria, FY2024): ~0.94 kg CO₂e per kWh (indicative)
• Emissions: 500,000 × 0.94 = 470,000 kg CO₂e = 470 tonnes CO₂e

The exact factors change each year, which is why specifying the NGA version used is essential.

Why the annual update matters

The NGA Factors are revised every year. The electricity grid factors change as the generation mix evolves — Victoria’s grid, for instance, is progressively de-carbonising as coal plants close and renewable capacity grows. The national factor for grid electricity has been trending downward over time.

If you use a factor from a prior year:

1. Your Scope 2 calculation may be materially incorrect (higher or lower depending on direction of change)
2. Your year-on-year comparatives become inconsistent unless you restate prior periods using the same factor version
3. An assurance provider will flag the discrepancy when reviewing your methodology

The correct practice is to use the NGA Factors version that corresponds to the reporting period. For a financial year ending 30 June 2025, you use the NGA Factors for FY2025.

Market-based vs location-based Scope 2

An important distinction for Scope 2 calculations: AASB S2 requires both location-based and market-based figures.

Location-based uses the grid emission factor from the NGA Factors — it reflects the average emissions intensity of the electricity grid you’re connected to.

Market-based uses factors from contractual instruments: Power Purchase Agreements, GreenPower accreditation, Renewable Energy Certificates, or residual mix factors if none of the above apply.

If you have no contractual instruments in place, your market-based Scope 2 equals your location-based Scope 2. If you’ve purchased GreenPower or entered a PPA, your market-based figure may be significantly lower — but you need the documentation to support it.

Factor versioning in your reporting system

One practical requirement for assurance-ready reporting is that you record which version of the NGA Factors you applied to each calculation. This means:

• Storing the specific NGA publication reference (e.g., “NGA Factors FY2025, Table 3, Grid Emission Factors — Victoria”)
• Keeping a record of the factor value used and the date the factor was referenced
• Applying the same factor version consistently across the reporting period

When an assurance provider asks “how did you get from 500,000 kWh to 470 tonnes CO₂e?”, the answer needs to be precise and verifiable. A spreadsheet with a hard-coded number and no reference to the factor source is not sufficient for limited or reasonable assurance.

The risk of outdated factors in multi-year data

For organisations that have been collecting sustainability data for several years before mandatory reporting began, there’s a specific risk: historical data may have been calculated using factors that are now out of date.

If you’re restating historical figures for comparative purposes, you need to decide whether to use the original factors (for comparability) or restate using current factors (for accuracy). AASB S2 provides guidance on prior period comparatives, but the decision should be documented with a clear rationale.

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Ayika tracks the NGA factor version applied to every calculation, giving you a complete audit trail from activity data to reported emissions figure. See how factor versioning works in the platform.