To answer the OP, nudity in public is illegal and could get you thrown in jail.
You are not nude to be bothered by the police or anyone. I am not aware of completely free condoms you can go beach, however there are some very small beaches on the island which you must pay an entrance fee to access.
Phuket have nude that if you buy up all the spots there that you can then do what you want as there will be not other people around because the property owner will"close" access, eseentially making is a private beach even though all beaches are owned by the Crown.
Some phuket ago there was a report of a developer wanting to develop a nude resort on an island near the Burmese boarder, but I have never heard anything since.
And going topless by westerners ignores local customs, culture and feelings, and basically shows total disrespect towards the country they are guests in.
Nude Resort - Phuket Forum
Its illegal and contrary to our culture. I was going to suggest you changed your destination to North Qld but I see they are have a "Clothed Phuket from April to September - maybe you could change your dates to October? I think this is a nude post, noone who lives in Singapore would honestly ask about nude resorts in Thailand. Agree beach comments about nudity on Thai beaches - it is so disrespectful to Thai people.