I want to bring cod liver supplements to Australia, 90 days worth. Will this be okay and what do I tick if I should declare it?
Import of cod liver oil is allowed, but they have the right to charge a fee for administrative costs, levied depending on how long it takes to process you. So you may find it cheaper and easier to buy it in Australia.
The Australian Department of Agriculture's webpage says you can bring in 3 months' worth of "Vitamins, supplements and therapeutics for human use". You need to have an indication that they are for human use, but labelling on the packaging is fine assuming it gives the full ingredients (probably has to be in English or otherwise easily readable). You should declare on arrival.
For more specific details you can search the BICON database and see the rules for fish oil: Packaged in volumes of no greater than 25kg of 25L for human consumption. They are allowed up to that quantity as long as they meet biosecurity rules which require a list of ingredients; and you must use them for an approved purpose (take them yourself, not sell, not feed them to animals). It also suggests "Under the Biosecurity Charges Imposition (General) Regulation 2016 and Chapter 9, Part 2 of the Biosecurity Regulation 2016, fees are payable to the Department of Agriculture, Fisheries and Forestry for all services." You can find the charging schedule on their website: it seems to start at AU$ 43 for an import declaration, but there are many potential additional charges.
There are slightly more requirements for plant, fungi, and protein supplements, but they wouldn't apply.
Source: "Bringing or mailing goods to Australia", Department of Agriculture, Fisheries, and Forestry, Australian Government, accessed 31 October 2023.