Otago Boys' High School head of music Rob Duffy said the two schools had been singing together for more than a decade, but this year the group had standout musical talents.
''A lot of them read music and play instruments, and that really helps,'' he said.
The singing competition, in the Auckland Town Hall from August 21-23, will showcase 24 of the top high school choirs from around the country. Only three choirs from the South Island reached the final this year.
The OGHS and OBHS choir will be the only one from Otago at the finals.
Soprano Calla Knudson-Hollebon (17), from Otago Girls' High School, said it was a joy to sing in the combined choir of 60 members.
''It's an empowering feeling when all the voices come together and the sound you're producing is unified,'' she said.
''You know you're all singing because you love it, and you can share the music with people around you.''
Tenor Marco Taiaroa (17), from OBHS, said the mix of male and female singers improved the output.
''When we come together, the music has a lot more room to move; there's more spectrum in the song,'' he said.
''Boys turn up to more practices when the girls are there too.''
Choirs would sing two songs in each of three categories of musical styles: New Zealand compositions such as Maori and Pasifika songs, classical compositions, and modern styles.
A male-only Otago Boys' High choir won a silver award at the Big Sing finals in 2012, and an OBHS and OGHS combination choir won a bronze prize in 2011.
By Robert Steven.
