Intriguingly, the Dolphins have only signed the 21-year-old on a one-year contract.
Niu, who played a number of games at fullback for the Red Hill-based side last year, but also battled with injury, was set to slide down the pecking order this season at the club.
Walsh was released on personal grounds, with the fullback deciding he was unable to relocate permanently back to Auckland.
Niu's release also comes despite coach Kevin Walters suggesting Walsh wasn't a sure thing to wear the Broncos' number one jumper, however, all reports suggested Niu would end up on the outer, with almost no chance that he would shift to the centres given the talent of Kotoni Staggs and Herbie Farnworth, who wear three and four for the club.
Released by the Broncos on Wednesday evening, it was reported the Dolphins would beat the Newcastle Knights for his signature, and that has now been confirmed, with the NRL's expansion side confirming he would begin training at Redcliffe immediately on Thursday afternoon.
Niu would have slotted straight into the number one jumper at the Knights with Kalyn Ponga forecast for a move to five-eighth, while at the Dolphins, he will now instead have to fight for a spot in the centres given the expansion club have signed Hamiso Tabuai-Fidow from the North Queensland Cowboys.
“Tesi has been in the Broncos system for a number of years and is highly regarded across the club," Broncos' coach Kevin Walters said in a statement announcing his departure.
“But we believe he will receive more opportunities elsewhere and it's an indication of our respect for Tesi that we have allowed him to depart.
“We wish Tesi all the best in the next part of his journey.”
The 21-year-old has played 32 games for the Broncos since his 2020 debut, and Dolphins' CEO Terry Reader said Niu would be an excellent addition to the club.
“We obviously know Tesi very well as he is a Brisbane-born player we have watched come through the grades and become an NRL regular and an international,” Reader said.
“We now have a strong list of outside backs with players such as Hamiso Tabuai-Fidow, Euan Aitken, Brenko Lee, Edrick Lee, Jamayne Isaako, Oliver Gildart and Robert Jennings all part of the Dolphins' foundation squad.
“This group will compete for spots in that first Dolphins' team to play the Roosters - and with the trials only four weeks away, that competition will start very soon.
“We have always said that January would be a busy month for recruitment and bringing Tesi into our squad is a great acquisition.”