The prop's comments come as the Broncos on Thursday waived the cooling-off period during which the prop could have changed his mind and stayed put.
Speaking via a video statement after the defending premiers touched down in London for the World Club Challenge, Haas defended his relationship with Maguire.
The 26-year-old also said he had come to the realisation he wanted to be a leader for his family and that he had more support for them in Sydney than he did Brisbane.
Haas is yet to front the media since signing a three-year deal with Souths on Sunday, which will begin at the start of the 2027 season.
The clear best prop in the world, Haas's decision to reunite with former coach Wayne Bennett shocked the NRL and looms as the biggest player transfer in decades.
In the days since, several rumours have spread around his desire for leaving Brisbane, including separate claims around differences in financial offers.
But the prevailing narrative remains that Haas had fallen out with Maguire, with multiple sources pointing to the coach's tendency to push his players to the limit.
"I can't believe the stuff with Madge," Haas said.
"I can't believe all the speculation and all the rumours that have been tossed up.
"It's far from the truth to be honest and that's me being honest and truthful and transparent.
"There's nothing wrong with us (Michael Maguire), we just won a premiership together and we won a State of Origin together."
It is not the first time Maguire has been the centre of such claims, with suggestions select Broncos players were unhappy with him midway through last season.
But despite his intense reputation, Maguire took Brisbane to last year's title after also winning trophies with New Zealand and NSW in 2023 and 2024.
"We've got a great relationship, and I enjoy working hard under Madge," Haas said.
"I'm a hard worker myself and that's what I pride myself on. The way we work is what I love and I love doing that day in day out.
"For everyone saying that kind of stuff it's not true and it's unfair to Madge and the Broncos as well."
Brisbane had been in negotiations with Haas's camp for the best part of nine months and had tabled a short-team extension for the prop.
The financial lure of renegade rugby competition R360, since postponed until 2028, had thrown another spanner in the works for the Broncos.
The South Sydney deal that stunned the NRL was only finalised over the weekend, with recruitment figures flying north with paperwork on Saturday.
The thought of Haas leaving Brisbane had for a long time seemed inconceivable, given he also acts as carer to his younger siblings.
"My decision was pretty hard to be honest. Obviously I love the Broncos and it's all I've known since a kid, since I came in from school," Haas said.
"The main factor for me was family. Nothing else.
"I felt that being a leader for my family is probably the best move for us to have a change and a fresh new start.
"I've got lots of support down there as well with my family and my agent as well, he's down there, he's like a second dad to myself.
"I need those kind of people around me especially now for my siblings and my daughter and for my son as well.
"We thought as a family that would be the best for us."
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