Stopover in Jnb enroute to Cpt: where do I clear customs / immigration?

[kpc]

I booked a saver J award Syd-Sin-Cpt for next year, and as the plane seems to stopover in Jnb before continuing to Cpt, I was wondering would I clear immigration and customs at Jnb? or remain in the transit area at Jnb and do the C and I thing at Cpt? or even stay onboard during the stopover at JnB. Is it the same on the return?? Any comment on the lounges Sq uses at Cpt and Jnb?
Thanks
PS: I know it is the old 777-200 from Sin to Cpt, but what can you do?

Edit: the stopover time in Jnb is only 1 hour both ways so my guess is that we would stay onboard.

[kpc]

No one know?

[Guy Betsy]

You clear customs and immigration at CPT as SQ has no traffic rights to pick up additional passengers in JNB so its originating SIN passengers only on the aircraft for the remaining sector to CPT. You remain on aircraft for both outbound and return sector.