Lin-Manuel Miranda had double the reason to celebrate landing the job of composer for Moana because he learned he was to become a first-time father during the same week.

The actor, singer and songwriter was in the middle of putting the finishing touches to his Hamilton musical in early 2014 when he was approached by Disney bosses about penning songs for the new animated movie.

Lin-Manuel admits it was "so surreal" to be invited to be a part of the Disney family as he was already preparing for an exciting new chapter in his personal life.

"It also happened to be the week we found out we were having a baby, so it was like, a life-changing week," he smiled on breakfast show Good Morning America.

The star, who had to keep the Moana news a secret until earlier this year (16), admits working on the Disney film gave him a welcome reprieve from the historical nature of the Hamilton production, which chronicles the life of Founding Father Alexander Hamilton.

"Moana was such a lovely break from writing Hamilton," he explained. "It was like, 'OK, I'm not gonna worry about the Founding Fathers right now, I'm gonna go spend some time with Moana, spend some time with (character) Maui and write those songs, and then when that was kicking my butt, I would be like, 'OK, I'm gonna go back into this (Hamilton)', and so they were opposing muscle groups in my brain."

Moana follows the tale of rebellious teen, voiced by Auli'i Cravalho, who sets sail on a mission to save her Polynesian island with the help of demi-god Maui, portrayed by Dwayne 'The Rock' Johnson.

The film already has a fan in Lin-Manuel's son Sebastian, now two, and he is hopeful his boy will take away a few lessons from the lead character.

"She (Moana) loves her family so much, but she's also fiercely independent," Lin-Manuel said. "I hope it teaches him to be brave and to be courageous and teaches him to fight for the things that matter to him."