The rules regarding separating board directors when engaged in air travel, are not set in any law in Australia. These are rules set by the board of directors, and every company has a different set of rules.
The Sundance Co corporate governance rules, did not have any ruling included, that involved separating directors into different flights when travelling to the same destination.
The bottom line is, they had more chance of being killed, travelling all together, in a Congolese taxi to the airport, from their hotel.
Perhaps they would have been better advised, to have some board ruling about the competence of the airline they used.