Umaga has been cleared to play after overcoming an ankle problem, and Aussie skipper Gregan and Sharpe believe taking on the All Blacks' finest will only enhance the contest.
"It doesn't surprise us that Umaga is fit," Gregan said.
"We were prepared for him being passed fit. He has done a lot of work this week and got himself right."
Sharpe said it was "great" that the dreadlocked skipper would lead the All Blacks.
"When we beat these guys we want their best team on the paddock," Sharpe said confidently.
The hosts will be forced to field two debutants in full-back Drew Mitchell and winger Mark Gerrard in what is billed as a crunch encounter for both sides.
The Wallabies have lost their last two matches and New Zealand their last one and victory is vital for each side if they wish to claim the Tri-Nations.
Umaga's inclusion is a huge boost for the visitors with his Wellington and Hurricanes team-mate Conrad Smith now set to miss out completely.
Coach Graham Henry has named utility backs Luke McAlister and Leon MacDonald on the bench as cover.
Injured half-back Byron Kelleher has returned to New Zealand, allowing another Hurricane, Piri Weepu, to join the team at number nine.
He makes only his second start, with Kevin Senio moving into the 22. Weepu will face the toughest head-to-head test rugby can offer, in the shape of Gregan, the world's premier half-back.
"I had better come with my A-game or I might get shown up," the 21-year-old told the Dominion Post.
"He (Gregan) has been around a long time. With that many caps comes experience and things like that. He has got all that on me, but hopefully I'll just focus on what I have to do and things will go well."
Weepu is one of four All Blacks starters who have never played against the Wallabies, but is eager to add his name to the illustrious roll call of Bledisloe Cup heroes.
"They are pretty much our rivals and we are their rivals," he said.
"They have come from South Africa, losing two games over there. They will be wanting to prove a point here.
"If they want to pull off a win on their home turf, they'll have to come out to play."