The Nova food processing classification system is increasingly used in dietary surveillance, epidemiological research, and policy(1). Ensuring a standardised and transparent application of the Nova system is critical to generate robust, replicable and comparable data. This is particularly relevant for the retrospective classification of dietary instruments not designed to assess Nova food groups(2). The recent release of the 2023 AUStralian Food and NUTrient Database (AUSNUT)(3) provides an opportunity to outline a methodological framework for addressing these challenges. This study aims to describe the systematic approach to classify foods and drinks within the AUSNUT 2023 according to the Nova system. The AUSNUT 2023 contains 3,741 food items (i.e., foods, beverages, ingredients and dietary supplements) to support dietary assessments of the 2023 National Nutrition and Physical Activity Survey and the National Aboriginal and Torres Strait Islander Nutrition and Physical Activity Survey. A methodological framework was developed to classify AUSNUT 2023 items into the four Nova groups, i.e., unprocessed or minimally processed foods (G1), processed culinary ingredients (G2), processed foods (G3) and ultra-processed foods (UPFs, G4). The three-step iterative approach included: 1) identifying handmade preparations through a series of workshops with five experts in the Australian food supply and dietary practices, using AUSNUT information on food name (to identify food sources: homemade, commercial, takeaway/restaurant, fast food chain, purchased frozen, ready to eat/drink, canned/preserved, dry powder, unspecified), food description, and sampling details; 2) disaggregating handmade preparations into underlying ingredients using standardised AUSNUT recipes; and 3) assigning single food items to Nova system groups and subgroups by independent experts, using information from AUSNUT Food and dietary supplement classification system, food source information, 2023 Australian food supply (using Mintel database) and supermarket websites. Summary statistics were generated for the distribution of items across Nova groups and subgroups. Of 3,741 items in the AUSNUT 2023, 25.2% of items were identified as handmade preparations and considered for disaggregation. Among the 2,832 single food items, 39.8% were classified as G1, 2.3% as G2, 12.5% as G3 and 45.0% as UPFs, with 0.4% of items not classified (e.g. vitamins). Among each Nova group, the top three subgroups with the highest number of items were: G1 - vegetables, red meat, and fruits; G2 - plant oil, table sugar/honey, and animal fats; G3, preserved plant foods, cereal-based processed foods, and cheese; and UPFs - confectionary and cereal/nut/fruit bars, pastries, buns, and cakes, and sauces, dressing and gravies. This study provides a systematic and transparent method to apply the Nova system to AUSNUT 2023. The methodological framework and corresponding classification are key resources for advancing UPF science, monitoring, and policy in Australia. It may also serve as a reference for transparent reporting of Nova classification in Australia and elsewhere.