It doesn’t matter. Volunteers need a work permit. Seriously. However, Immigration doesn’t look for infractions unless someone reports you. Then, they are obligated to investigate.
Not sure any one person has experience with all 223 Immigration offices. Besides, the policies probably change from time to time, maybe as each new manager takes over or changes their mind.
Go to Immigration just before your stamp expires, asking for another extension. You won’t get the extension, but they will give you 7 days to leave the country.