"One of our My Kitchen Rules New Zealand social posts went out earlier than it should have," explained a spokesperson.
"We apologise to our viewers who saw the finale result ahead of time. We are working to find out how this error occurred and we are grateful to the programme's fans who are helping us keep this spoiler under wraps."
Fairfax reported the video had been watched more than 600 times before TVNZ realised the problem and deleted it.
TVNZ is yet to comment on the mishap.
But fans took them to task on Facebook.
"Cheers for ruining it. I was looking forward to watching it tonight," wrote one.
"I'm absolutely disgusted this has been released. Someone needs firing," said a second.
"No need to watch it now," wrote a third.
It's not the first time TVNZ has spoiled an episode of My Kitchen Rules, with the network apologising for airing a promo during an ad break in 2015 that showed unscreened events from a future episode of My Kitchen Rules Australia.
"Guys, we're so sorry about last night and we're making sure it won't happen again!" the network said at the time.
Tonight's finale is a cook-off between Christchurch's Heather and Mitch, and Wanaka's Chris and Bex, with guest judges Nadia Lim and Ray McVinnie joining Manu Feildel and Pete Evans.
TVNZ has already confirmed the reality show will return in 2018.