NSW have won two of their opening three matches and former rugby league international Tahu has been impressive following his recent switch to the 15-man game.
But the hamstring strain means the 28-year-old will miss the local derby match against their fierce rivals, with talented youngster Alfi Mafi taking his place.
"I'm really disappointed to miss this game because the local derby with the Brumbies is one of our biggest matches," Tahu said.
"I was feeling a bit tight yesterday, so I went for an MRI which has shown a strain.
"It's too big a risk to play this week, so I'm making sure I do all the right things in rehab so I'm back on deck for the Crusaders match after the bye."
NSW coach Ewen McKenzie revealed the decision to rest Tahu was made easy because the Waratahs have a bye the week after the Brumbies match.
"It's not a massive injury but it could be if we pushed it now," McKenzie explained.
"With the bye week next week there's a chance to knock it over. We miss one game but if we push that we feel we could make it worse, so we just thought we'd take the advantage of the bye and have him right for the Crusaders week (in round six).
"He'd love to be playing but we don't want to turn it into a bad injury."
McKenzie has been pleased with the Waratahs' start to the season, but admitted there is still plenty of room for improvement.
"We're running fourth in the comp so we're not going too bad," the coach said.
"I don't think we've played our best football - we think that's still ahead of us.
"Whether that comes tomorrow night I don't know but it's a local derby and that certainly brings out the emotions and the enthusiasm.
"I think the guys are in a pretty good mood at the moment so (hopefully) we can translate that into a good performance."