Dual international Williams revealed the agreement on Monday at a media conference where he announced he was taking up "an offer that I pretty much couldn't refuse" for a stint with Japanese rugby club Panasonic Wild Knights after completing his contract with Super Rugby leaders the Chiefs after their finals campaign.
He admitted it would be tougher than he'd expected to leave New Zealand rugby after enjoying a stellar season with the Chiefs which has elevated him to first-choice All Blacks inside centre ahead of long-serving World Cup star Ma'a Nonu.
While he declined to name the NRL team he's to join next year, 26-year-old Williams has been strongly linked to the Sydney Roosters for several months.
"This is due to a handshake agreement made a few years ago - before I came back to New Zealand - with an NRL club," Williams told a media conference in Hamilton.
Williams, who controversially walked out on the NRL's Canterbury Bulldogs mid-season in 2008 to switch codes to play rugby in France, said nothing had been signed with his 2013 NRL club and he wasn't in a position to elaborate.
Asked if he would like to stay with the Chiefs, he replied: "In a perfect world yes, but it's a pretty difficult situation.
"I just have to make the most of that.
"At this stage, if all things go well on their part, then I will be playing league.
"When I go there, I can't have this in the back of my mind. I have to start fresh and give them my all, which I will."
As for whether he regretted the handshake, he said: "It's a tough one."
He hadn't expected to enjoy his time with the Chiefs as much as he did but it wasn't an issue he could dwell on.
Williams admitted to nerves when he made his announcement at the Chiefs' headquarters.
He brushed off the brickbats that might come his way over his decision, saying be couldn't dwell on negatives.
Despite speculation that he could return to New Zealand rugby in time for the All Blacks World Cup title defence in 2015, Williams said he hadn't decided on his sporting future beyond next year.
If he did return, it would be with the Hamilton-based Chiefs, "even if there was no money here".
A member of New Zealand's Rugby World Cup winning effort last year, he hoped it could be possible for him to return to the All Blacks one day.
"But in saying that, I'm not going to hold my breath," he said. "There's some great players here. Those players deserve all that they get.
"I've chosen my path and now I've got to walk it."
All Blacks coach Steve Hansen said it was tough losing a player who was starting to fulfil his promise in the 15-a-side game.
"It is a shame and disappointing that New Zealand rugby is losing him," Hansen said.
In the meantime, Williams could play in next year's rugby league World Cup.
He said he was open to representing the Kiwis, for whom he played seven Tests before his code switch, but said his form would need to justify selection first.
The Japan club contract allows Williams, who is the New Zealand PBA heavyweight boxing champion, one fight during the season.