Imaging Resource tested the weather sealing of multiple cameras including the new Sony A7rIII:
Given that the A7R III has a significant number of environmental seals (see the screen capture to the right, taken from the Sony website), we were surprised that it had as hard a time with our test as it did – particularly given that all of the other cameras passed it easily.
While the A7rIII sealing seems to be improved over the predecessor it’s disappointing to see water going inside the battery compartment. This is really something that should disqualify the camera as “weather sealed”.
I would say the A7rIII is sealed if you keep it for a short time under light rain. I would not risk to keep the camera in much harsher conditions for a long time.