The script indicates that the T-800 Terminator punched clean through the T-1000's 'head'. That being said, fluidity is an integral function of the liquid metal, hence why bullets hitting it cause craters.
It comes off the wall straight at Terminator, who smashes his good
fist into its face. The pile-driver blow buries Terminator's fist
almost to the elbow.
But the T-1000's head morphs in a split-second into a hand which grips,
geometry shifting faster than we can follow.
Here's the same scene from the official novelisation.
To ward it off, Terminator smashed his good fist into it. The
pile-driver blow buried the cyborg’s fist almost to the elbow in the
But the T- 1000’s head morphed in a split-second into a hand that
gripped Terminator’s wrist, and the head “emerged” somewhere else, the
geometry shifting faster than Terminator could follow. The T-1000
slammed Terminator into a large conveyor motor, jamming his arm into
the moving works.