There are 3 steps in Vote counting.
1. Box Count - Counting the votes inside the box and verify with the SPO's report. If it is Ok, then moves to 2nd step.
2. Party Count - Categorize votes according to parties and remove rejected votes. Then check the sums with box count. If it is Ok, moves to 3rd step.
3. Preferential Vote Count - This will do party wise. Then categorize ballet papers in to 5 categories namely, which have marked 1 Preferential Vote (PV), 2 PV's, 3 PV's , No PV's and more than 3 PV's. Then remove No PV's and more than 3 PV's.
Now there are 3 type of ballet papers are remaining. 1PV marked, 2PV marked and 3PV marked. Get each set and do the counting. It use tally sheets which contains No. of the candidate and counters check each ballet paper and mark PV's in the tally sheets. (Counters have to double check the tally sheets and signes as prepared by and checked by)
Finally, Assistant Returning Officers (Executive Level) and Chief Counting Officer (Class I Executive Officer) recheck tally sheets and make the final summary. If there is an error, final summary will never tally and the officers have to find the error until it balanced.
I have to emphasize, since the system is so tide, there is very very less probability to cause an error. It is really hard to cheat this process. No way.