I dont think that is correct. I recently paid a small (but significant) lump sum into my bond. I could see the daily interest charged coming down a bit. Obviously this lead to less interest being tagged on at the end of the month (I made the payment mid-month).
However, the amount automatically deducted from my account (the re-payment) still hasn't changed. I think they do a readjustment every few months and, perhaps, on your request.
Correct, your lump sum adjusts the capital amount. However, when the repo rate is adjusted, the bank can keep their prime interest rate for its current homeloan clients on the higher interest rate as long as it wants (which is exactly what Nedbank did in the last cycle with myself and tim).
There was no benefit passed on to me. Your lump sum payment changed the capital amount meaning your loan term is shortened. You pay the same rate of interest, irrespective of the amount left to go, so you basically enjoy a savings advantage as your loan term was lowered, thus meaning you will pay less in the end. Interest is a cost and goes to the bank anyway, and the pro-rata adjustment by some banks are passed on to the clients.
The other banks do not adjust their interest for those clients until the start of the next cycle, therefore their costs are the same and they effectively profit against the repo rate coming down. It's not really something to be frowned upon as it is their right, however as some banks do pass the benefit on to their clients it may well be construed as a bit unfair, but then again, who really wants to go through the schlep of moving a bond, let alone in my case 5.