If you still have your warranty you should head down to the nearest Sony official repair shop otherwise you'll need to follow these simple steps :
1- Turn off your Z3+
2- With a hair drier heat up the back of your phone for about 10 to 15 minutes
3- With a suction cup lift up gently the back cover and using guitar pics or credit cards remove the adhesive (note that your phone is no longer water proof at this point)
4- You'll clearly see the components of the phone ,all you have to do is remove the battery from the motherboard then do the same to the camera
5- Repeat the steps above in an opposite order and here you go.
I've had the same issue as you and I handled it as such, nonetheless I'm not responsible for any damage you may cause your phone, it's all on your own risk.
