"I often walk past the Powerstation on my morning walk up Mt Eden, and every time I think how I really love that room. I've always had a good time there, and really have quite a history there. And I kept thinking, 'everyone is coming home, wouldn't it be fun just to do a big old family bash at the Powerstation?'
"So I emailed [Liam and Elroy] a couple of days ago, just as they were at the airport in New York, about to come home, and they just went 'yeah!"'
They will all be rehearsing together for their appearance at the Falls Festival in Australia over New Years, so it seemed like a good time to make a local appearance too.
"It's funny because you don't get to play these kind of things in your home town that often. Somehow they seem more appropriate overseas, but that's weird, why would they be, so it's great to be able to do this" Liam says.
They also felt like it would be appropriate to give the proceeds to the people who need it most at this time of year, and give the occasion some extra Christmas spirit.
"Whenever we've talked about it in the past, it hasn't been a money making scheme, it just seemed like a fun thing to do" Liam explains. "And we thought, why not make it for something bigger than ourselves."
"It's hard to find meaning in Christmas without some involvement in the wider community" Neil adds.
"It's always going to be the case that whenever some people are doing well, other people won't be doing well in the community, so it's good to remember that, and it just puts a nice complexion on the night."
It seems like a win not just for the family, and the City Mission, but the fans too. They'll have the privilege of hearing a bunch of news songs from Liam and Neil's upcoming new records for the first time.
"That's the other exciting thing about doing it, it'd be great as a big old romp, but it'll be really fun to share those new songs at home. Some of them probably the first time they've ever been on stage. Tim and I have been writing a few recently too, and there's one I'm gonna try and persuade him that we should do."
The rest of the set will be an assortment of songs by Tim, Neil, Liam, Finn Brothers, maybe even some Split Enz.
"Anything and everything we can remember how to play really, up and down, through the ages" Neil laughs.
"People occasionally request some quite obscure songs from the back catalogue, and it's not so much I don't want to play them, I just can't remember them. Someone requested one in London the other night, and I actually thought about it, and even when someone in the crowd sang some of the first verse to me, I still couldn't remember it. Terrible. I think sometimes when you write new ones, you have to eject a few old ones."
Liam reckons they'll remember plenty though.
"We played a gig with Dad in New York recently, and Dad hadn't given us much of a brief at all on what we were supposed to play, and there was an orchestra there as well, so there was all sorts happening, but ultimately - and I keep forgetting this and end up feeling a bit insecure - something happens when we're all up there. You can kind of just trust your muscle memory, and instincts, and it all just seems to happen."
"Liam managed to invoke the sound of a truck during one of the new songs called Lights of New York. I couldn't work out right until the end what it was, but then I realised it was Liam" Neil smiles.
"And then I managed to play about two minutes of feedback without realising it was coming out of my amp" Liam laughs. "So people can look forward to some of that."
"It'll be a bloody good time, I can absolutely be sure of that" Neil nods. "Because whenever we have family get-togethers along these lines, when we've done it at home, we really know how to have a knees up when we get going. There's been some wild nights, so it's going to be great."
• Saturday, December 21 at the Powerstation.
• Tickets through Ticketmaster
