"The world is definitely his oyster. He's already a fantastic talent ... but for him professionally it's amazing. He'll definitely get more work out of this if he wants it.
"What you've got here is someone who has got a fantastic ability to write a song and he's on the money popularity-wise and he's a young song-writer."
Winning an Oscar for Disney would make the company keen to hire McKenzie again, Mr Markson said. "They use so much original music and now they'll be over the moon with him ... and so if he wants to do more writing, Disney already know who he is."
The French silent movie The Artist and Martin Scorsese's Hugo won five Oscars each. And Meryl Streep ended a 29-year drought to win the best actress award.
McKenzie, in a brief acceptance speech, spoke of watching The Muppets on TV as a child in New Zealand: "I never thought I'd get to work with them."
He added: "I was genuinely starstruck when I got to meet Kermit the Frog, but once you get to know him, he's just a normal frog. And like many of the stars here, he's a lot shorter in real life."
McKenzie broke into Hollywood in the television comedy Flight of the Conchords, the show he wrote and starred in with long-time collaborator Jemaine Clement.
He said, jokingly, that splitting from Clement appeared to have worked in his favour: "It seems to have gone really well.
"But I'm looking forward to writing with Jemaine in the future. Because I'll be able to pull out the Oscar card and say, 'We should play this chord ..."'
McKenzie thanked his parents Peter McKenzie and Deirdre Tarrant for "never telling me to get a real job".
His mother said from LA - where she was baby-sitting his children Vito and Leo - that the family were "a bit overwhelmed ... we're all in speechless mode".
"[Bret] rang and all he could say was, 'I can't believe it, Mum, I can't believe it.' The city is just madness. Everyone's famous, and then there's Bret. And now he's famous, so it's all good."
Ms Tarrant said she felt indescribably proud seeing her son collecting the golden statue.
McKenzie also thanked his wife, Hannah Clarke, who was in the audience.
The other NZ nominees, Daniel Barrett, R. Christopher White, Joe Letteri and Dan Lemmon from Weta Digital, missed out on the Best Visual Effects Oscar. They were nominated for their work on Rise of the Planet of the Apes.