Lock Van der Merwe injured his ribs during the 39-20 loss to Australia on Saturday, while flanker Brussow is sidelined with a hamstring strain.
Full-back Gio Aplon is suffering from back and shoulder injuries after falling awkwardly against the Wallabies and is a doubt for the contest against the All Blacks in Wellington.
The Springboks are missing over 20 front-line players and were further weakened when lock Johann Muller returned home due to a hamstring complaint.
Muller's place in the squad was taken by uncapped second row Gerhard Mostert, but Van der Merwe will not be replaced as coach Peter de Villiers says his side have "run out of locks".
South Africa were well beaten in their Tri-Nations opener against the Wallabies and now have to travel to the odds-on tournament favourites with severely depleted ranks.
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The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
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The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
