Anyone know the differences in the two grading systems?

A friend has just come back from South Africa where (as a complete
rafting novice) he was running Grade 5. The kayakers supporting the
raft all went off to do a Grade 6 run in the afternoon. The numbers
seem a bit high to me.

Any ideas? Is a grade 6 raft-rapid actually a grade 4/5 kayak-run?

Zatt.