Dave's right, clean the bike to establish where the oil leak is coming from.
My first thoughts are that it could be the two stroke oil leaking in to the crankcases. This then mixes with the gearbox oil and will eventually work it's way outta the overflow pipe.
Sounds serious I know, but is a fairly common NSR fault and a fairly simple fix.
What happens is the 'O' ring on the 2-stroke oil pump slips down it's shaft and allows the oil to pass into the cases. What you need to do is remove the oil pump from the cases. Watch out for the small washer at the bottom of the shaft.....it'll most likely fall off when you remove the shaft or might remain in the cases.
You sould now see the 'O' ring has slipped down. Clean it and the shaft up, removing all the oil. Then using a bit of sealent, 'stick' the 'O' ring back up in to the housing at the top of the shaft.
Put it all back together and it should be OK.
One way to quickly check if it is the 2-stroke leaking into the cases is to open the gearbox oil dipstick whilst the bike is on it's sidestand. If over full, oil will gush out, so get something to catch it in before doing it.
Hope this all helps and good luck.