Same requirements as any piece of equipment.
1. Is it maintained in a sanitary manner that precludes contamination from dust, condensation, weather, pests..
2. Is it cleaned at a frequency necessary so that it does not become a source of contamination.
3. Is it protected from intentional adulteration.
General GMP's: "The plant and facilities shall take the proper precautions to protect food in outdoor storage or processes such as receiving, initial product washing or bulk fermentation tanks."
I would reach out to a brewery or dairy under the scheme and see if they can help you with what their auditors have required in the past. You can use the public supplier search: https://www.ac.parkc...lcares?id=40825 and search for companies under category 10 and reach out to see if their QA person is willing to chat about how they maintain outside raw milk tanks (if they have them) while under the scheme.
The PMO and dairy GMP's may have specific guidance in general that you can use to validate your setup.