Australia's Ben Simmons says his mental health struggles began well before the 76ers' last NBA season ended and didn't believe he'd be able to overcome them in Philadelphia.

Simmons is eager to begin moving on with the Brooklyn Nets, though isn't sure yet when he will join them on the court.

Nor was he sure how things got so bad in Philadelphia.

"If I knew, I would tell you everything," Simmons said.

"But there's just a lot of things internally that had happened over time, and it just got to a place where I don't think it was good for me mentally.

He spoke at the Nets' practise centre, less than a week after the 76ers traded him to Brooklyn as part of a package for James Harden.

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The former number one overall pick hasn't played since the 76ers were beaten by Atlanta in game seven of the Eastern Conference semi-finals.

Post-game comments from teammate Joel Embiid and coach Doc Rivers after Simmons played poorly were considered the catalyst for his trade request.

But Simmons said that wasn't the case.

"That was earlier than that series or even that season that I was dealing with and that organisation knew that," he said.

"So it's something that I continue to deal with, and I'm getting there and getting to the right place to get back on the floor."

Simmons said he hoped to be ready to play by the time the Nets visit the 76ers on March 10.

He thanked the 76ers organisation and fans for their support and said he had spoken with some teammates and personnel — though not Embiid — after the trade.

He also spoke that day with Kevin Durant, whom he will join along with Kyrie Irving in a line-up the Nets hope will be good enough to win the Eastern Conference.

"I think it's going to be scary," Simmons said.

That was also the expectation for his partnership with Embiid, the 76ers' All-Star centre.

They led Philadelphia to the best record in the East last season, but Simmons was already dealing with personal challenges he felt required him to leave the organisation.

"I think I just wasn't in the place there to do that and a lot of things had happened over that summer to where I didn't feel like I was getting that help," Simmons said.

He was seen smiling when he watched the Nets on Monday beat Sacramento to snap an 11-game losing streak.

Asked if it had all been an excuse to seek a trade, Simmons said: "People are going to say what they want. I've had some dark times over the last six months and I'm just happy to be in this situation with this team and organisation."

AAP