Smit, who was suspended by the IRB for six weeks for an incident during November's Test match against France, was expected to return for the Sharks in the March 4 match against the Waratahs in Sydney.
But an injury sustained in training means he may now miss several weeks of the competition.
Coach Dick Muir told the Beeld newspaper: "We hope he'll be able to start training with us from March 5, but that's the earliest we can expect to see him."
The Sharks will also be without three other Springboks for the start of the tournament - Adrian Jacobs, Dean Hall and Butch James - as injuries have forced them out of the first few games.
Meanwhile, Cats prop Baksteen Nell has retired from all competitive rugby due to personal commitments.
"I want to spend more time with my family and my business that I started two years ago," said Nell.
Cats coach Frans Ludeke reluctantly accepted his decision, but said he would miss the big prop's scrummaging abilities.
"I understand Baksteen has more responsibilities with the birth of his second daughter and we fully support him in his decision. He will be missed."
The Cats have re-signed veteran former Newcastle prop Marius Hurter, 35, to cover for Nell.