Passing through JNB on way to CPT - immigration?

Hi,

we are hoping to get onto a flight to JNB and then on to CPT today. I know I'm a bit late, but maybe one of you can send me a quick answer. Do we have to go through immigration in Johannesburg or in Cape Town?

Thanks:)

point of entry, Joburg, then thru to transfer over to domestic terminal (long walk)
no valuables in checked bags
make sure you have 2 consecutive facing pages in passport (not addendum pages and not back to back)