Serial killer Ivan Milat is on a hunger strike after sawing off his little finger with a plastic knife on Monday.

Milat, 64, was placed in an "observational cell" at Goulburn's Supermax jail yesterday after returning from hospital, but has refused all meals, a Department of Corrective Services spokeswoman said.

"I don't know if he gave a reason or discussed it with anyone ... but you can call it a hunger strike," she said.