Thanks for your question and I apologise about our delay in responding to you.
Water for washdown stations in rural areas of Cape York wouldn’t have any mains water supply available, so you will have to consider use of surface water, groundwater or rainwater. However use of surface water for example (like a river) will be very site-dependent, so you might consider groundwater using a bore, or if rainwater is relied upon then having an alternative or backup water source for the dry season if the water captured in the wet season won’t last the entire dry season.
Most of the time all washdown facilities recycle water internally.
Be sure to look at the photos of an existing (but broken down) washdown station on Cape York in the photo gallery on the Design Area 1 webpage. Unfortunately I don’t have information about the type of water source that this example station uses but will try to find out for you, except noting that a significant amount of water is required for it to run. So based on how your solution works, and your calculations on how much water is needed to wash one average vehicle, and how many vehicles pass through each day and the water lost in the process, you will be able to determine how much water is required.
In terms of specific procedures for vehicle users/rangers on Cape York, this was the only washdown station we knew of in the entire Cape. So I think it unlikely there will be any Cape York-specific procedures you will be able to find and we aren’t aware of any, so best to use the QLD ones you found.
Hope that helps and best of luck with your project,