The first three episodes of the X Factor Australia were screened in New Zealand in a three and a-half hour programme on TV3 on Saturday, and the pair are amazed by the attention they are getting on mainstream and social media.
"We knew there was going to be a good reaction, but I think we both underestimated the scale of it," Finley (26) said.
"We've never experienced anything like this. It's been crazy."
The pair's mother Kathleen, a stalwart of the performing arts in Wakatipu, says they are "pretty overwhelmed by the level of interest and support they're getting from New Zealand.''
But she was not surprised by the impression they had made, as they had spent the past few years ''honing their craft'' and had chosen a song that complemented their voices, she said.
''They try to create a unique sound for themselves. They strip it right back and put their own bit of magic into it.
''I'm really quite proud of them.''
*Warning: This clip contains explicit language
The pair attended St Joseph's Primary School in the resort before going to boarding schools in Oamaru and Timaru.
They went on to further study in Christchurch; Finley graduated from the New Zealand Broadcasting School while Isla studied musical theatre at the National Academy of Singing and Dramatic Arts.
In 2014, shortly after coming third in Queenstown's Songstars original music competition, they moved to Melbourne to focus on their music.
The shift paid big dividends, and the pair quit their normal jobs to perform full-time at weddings, events and on cruise ships as Brentwood Duo, Mrs Brentwood said.
They had become adept at marketing themselves, to the point where they had to opt out of other work a year ago to keep up with demand.
Although unable to give away too much about their future on the show, it was safe to say they had progressed to the intensive training sessions known as "boot camp".