The American told reporters on a video call that his drivers were raring to get going.
"He (Piastri) is now going into his fourth year. Lando has a lot more grands prix than he does so if you look at the development of Lando over that time, Oscar's on a similar trajectory," Brown said.
"So he's in a good place, physically very fit, excited, ready to go."
Last Australian champion was in 1980
Piastri, who debuted with McLaren in Bahrain in 2023, can become Australia's first champion since Alan Jones in 1980.
While Piastri took his first win in his second season, Norris had to wait until his sixth. Both won seven times last year.
Brown said he had spoken a lot with the Australian over the European winter break and expected the 24-year-old, championship leader for much of 2025, to pick up where he left off.
He said the discussion had been all about creating the best environment for him and what McLaren needed to do to support him.
Brown said Piastri had spent time in the simulator and, in response to a question about lingering sentiment in Australia that McLaren favoured Norris, "he knows he's getting a fair shake at it".
"You win some, you lose some. Things fall your way, things don't fall your way," added the chief executive.
Pre-season favourite
Brown said Norris' confidence level was also very high.
"He's highly motivated and it's our job to give him and Oscar the equipment again to be able to let them fight it out for the championship," he said.
"If we can do that, I think Oscar and Lando will both be in with a shot."
Mercedes' George Russell is the current pre-season favourite after an initial shakedown test in Barcelona last month.
Norris can become only the second Briton to take back-to-back titles after seven-time champion Lewis Hamilton, who won four titles in a row with Mercedes from 2017-20 as well as two together in 2014 and 2015.
The only other multiple British world champions are Jim Clark (1963, 1965), Graham Hill (1962, 1968) and Jackie Stewart (1969, 1971, 1973).
"I think there are some drivers that say 'I've done it. Now I'm done'," said Brown. "And then you have drivers like Lewis Hamilton and Max Verstappen and Michael Schumacher who go 'I've done it once, now I want to do it twice and three or four times'."
He reiterated that both remained free to race and said decisions would be taken strategically as and when they arose.
"We feel like we'll be competitive. The top four teams all seem very competitive. Very early days but indications that we will be strong," he added.