Digging around this picture came up showing the Mulbera at speed. I must admit that, being built in 1922 and, at that time almost certainly burning coal in her boilers, I would have expected to see a bit more from her funnel.

As she was in service until 1954 it does make me wonder whether she might have been converted to burn oil, which would certainly give her cleaner combustion.
SA Oranje definitely was an oil burning ship so, to be expected would have a noticeably cleaner exhaust.