Are hospitals free in Thailand, and what happens if you turn up without insurance?

No. Australian government advice states there is no reciprocal healthcare agreement, that tourists pay for all medical care, and that hospitals generally require a deposit and confirmation of insurance before admitting you. That last detail is the one people are unprepared for, because the conversation about payment happens before the treatment does. Worth adding that policies commonly exclude riding a motorbike and diving unless you add them deliberately, and those are the two activities most likely to put a visitor in a Thai hospital. Sourced here: https://meetlocale.com/guides/is-thailand-safe-to-travel-alone If you have been treated in Thailand, what did the hospital ask for before starting?