Smith is being rested after a heavy load he has carried during the season, while Frizell will be out for a month after tweaking his ankle in the last moments of last weekend's game.
However, coach Clarke Dermody said it was handy to have James Lentjes available to fill the role because he had been making an impact when coming off the bench.
They are expecting a fast game from the Waratahs. They only kicked the ball twice in the second half against the Crusaders. They were a young side that finished fast, Dermody said.
That was something the Highlanders were used to in their perpetually dry environment under the roof at Forsyth Barr Stadium. They wouldn't change the way they play, but would make a couple of small alterations for the Waratahs, he said.
The Waratahs were averaging four tries a game and were a quality side who lacked some execution at key times, which had let opponents off the hook.
The Highlanders' attitude was a simple one.
"We're expecting their best game and hopefully, we're good enough," he said.
The side enjoyed having the chance to play at home after last week's Covid-19 forced visit to Sydney to play the Rebels. They had been disappointed not to play in Queenstown as scheduled because it had been so long since they had played there.
At the same time, it was refreshing playing Australian teams who were already showing the benefits of competition against New Zealand sides, he said.
Dermody felt the benefits from the intensity of Sky Super Rugby Aotearoa had caught the Australians out at the start of the competition but they had adapted, and games were starting to tighten up.
“They're really fit teams. We've found every game, they have been pushing us hard toward the end of games.”
The side's defensive effort had lifted during the trans-Tasman competition to the point they had the best tackle completion rate of all sides. Dermody said that was down to the attention they had paid to their tackle techniques.
Having seen the Waratahs' ability to secure breakdown turnovers, the Highlanders had taken note. They realised that if they couldn't get quick ball from breakdowns, their game slowed down, resulting in them having to kick the ball away.
Dermody said the Highlanders' quick ball statistics were high, they aimed at three seconds per ruck, and against the Rebels, they had managed that nearly 70 per cent of the time.
"The knack of it is trying to slow the opposition down, so if we can make our tackles and slow that ball down that is more important for us," he said.