I've had this before, the new cable I fitted sucked oil out the box, but first I knew was when it dripped onto my pedals! Very dangerous as you can imagine. As said before, check breather on box and oil level
check the oil level if breather looks clear, undo large plug on the end of g/box, make sure your on level ground, you need 17mm allen key, put a container under hole and let oil drain until its level with bottom of hole
Undo the fill plug first obviously, as you don't want to drain it then realise the fill plug wont come out! If the oil pours out the fill plug, then thats when it will have been overfilled. When it stops draining there then its at the correct level. And obviously make sure its level on Axle Stands. Its a PITA trying to undo them when its just on stands though, its hard to get leverage on a breaker bar to undo them. You may need to soak the plugs for a couple of days in WD40/Plusgas.