Their ambitious programme, beautiful tone and well-directed pieces resulted in the Chapel Choir being named overall winners.
The school was the only one in the East Coast region to make the final 24, from a list of more than 200 nationwide.
Ms Tobin said being selected for the Big Sing National Finale was a proud moment for the choir girls and the school.
"It will bring our school's name to the forefront while allowing our girls to perform at their extreme best."
The last time Woodford House competed in the national event was in 1999 under the leadership of Gary Bowler.
Ms Tobin said many hours were spent selecting music followed by months of rehearsals, which prepared the girls for competing at a high level.
"I look for pieces that are challenging and that present the girls' voices at their best. I always have the mind-set that the pieces must be competition pieces but must also present our Christian values where possible."
When Ms Tobin selected a piece by Anthony Ritchie called The Heavenly Miracle, she was unaware that Ritchie would be an adjudicator at the East Coast regional competition.
"This added another layer of expectation on to the choir knowing they were going to be performing this very challenging piece to the composer.
"We worked hard to perfect his piece from the long, difficult phrases to the smallest of musical idioms and, as a result, we won the New Zealand category."
The Woodford House choir, which is made up of Year 11 to 13 vocalists, will perform five pieces at the The Big Sing National Finale in Dunedin from August 25-27.