This paper presents the draft design-hazard factor (Z) maps developed for the 2025 amendment to the Australian Standard Structural Design Actions: Earthquake Actions in Australia, AS 1170.4:2024. The peak ground acceleration (PGA) hazard map for an annual exceedance probability (AEP) of 1/1,500 from the 2023 National Seismic Hazard Assessment of Australia was used as the basis for the updated design-hazard maps. This map was subsequently adjusted through two primary approaches, such that: 1) a factor of 2/3 was applied to the PGA map to adjust the 1/1,500 AEP hazard such that the overall design values are consistent with the 1/500 AEP requirements specified in the National Construction Code; and 2) an adjustment was applied to compensate for differences between the AS 1170.4:2024 spectral shape factors, Ch(T), and the normalised probabilistic uniform-hazard spectrum (UHS) averaged over all Australian localities at a vibratory period of T = 0.5 s. The resulting hazard grid was contoured at 0.08 and 0.10 g and these irregular contours were subsequently fitted using regular geometries. Zones enveloped by these geometries were assigned a Z factor of 0.10 g (Zone 2) and 0.12 g (Zone 3), respectively. Consistent with the AS 1170.4:2007 (Amt 2, 2018), a floor Z factor of 0.08 g (Zone 1) is applied to the remainder of the Australian continental landmass. Spectral shape factors for periods between 0.0-0.3 s are also reduced from 2.94 to 2.5 (on Site Class Be) to reduce the short-period demand and to be more consistent with the average probabilistic UHS. The resulting Z values anchor the adjusted elastic design spectrum at T = 0.0 s and are intended to provide protection to structures from earthquake loading, with a primary focus on collapse prevention, for a notional AEP of 1/500.
Prepared for the Australian Earthquake Engineering Society National Conference, 20-22 November 2025.