The Block NZ: Previous winners Amy and Stu are back as Team Black

The contestants on The Block NZ: Firehouse were stunned by a shock announcement on Monday night's episode that saw a fifth team enter the fray.

Amy and Stu, the Gisborne couple that won the competition in 2018, returned as Team Black in an unprecedented move for the Kiwi renovation show.

Moving into the previously uninhabited apartment five in the fire house, Amy and Stu will be delivering a family bathroom along with the other teams.

"The whole game has changed," Mikaere said after the announcement.

Not everyone was pleased about the new additions to the game, especially considering Amy and Stu will be eligible to keep the $7,000 prize money from TSB if the other teams can't beat them.

"I'm not excited. Because we won't win, we can't," Stacy whispered to her partner Adam following the news. 

"It was always going to be a hard week and it's just got even harder," Adam said.

"They've done this before. They know when things need to happen on bathroom week to get it done."

To those who think getting a second crack at The Block is unfair, Amy's advice is to simply "enjoy the ride".

"I think the other teams were initially shocked and a little confused but then it becomes really exciting and we felt really welcome," she told Newshub.

"They are all such great people!"

While Amy originally had dreams of returning to the DIY show as a judge, she says partner Stu was the driving force behind the couple's decision to take on the challenge again.

The Block NZ: Firehouse airs Monday - Wednesday at 7:30pm and Sunday nights at 7pm on Three.