Williams, 26, told the conference his decision to play in Japan was the result of a "handshake agreement" with a Panasonic official before he even returned to New Zealand. It was with some regret he was leaving but he felt he had to honour the agreement.
"Panasonic made me an offer (in 2009) and I turned it down. Then they came back with an offer I basically couldn't refuse."
Williams said his "full focus" was on the Chiefs and helping his "brothers" in their bid to win their first Super Rugby title. He said he had enjoyed his time in Hamilton so much that, ideally, he wouldn't go to Japan.
"In a perfect world, yes [I would stay]. It's a pretty difficult situation but I just have to make the most of it."
Williams told the conference he would be back in the NRL next year but refused to confirm at which club because he was yet to sign a contract.
The former Canterbury Bulldogs second rower has long been rumoured to be returning to league with crosstown rivals Sydney City Roosters, with a view to switching back to rugby in 2014.
Once he completes the switch back to league, Williams said he would be available for selection to add to his seven caps for the Kiwis.
"If all goes well and my body goes well and I'm playing good footy. I wouldn't want to be picked for any side just because of my name."
Williams walked out on the Bulldogs mid-season to play rugby union with French club Toulon, a move which saw him voted as Australia's most hated person - ahead of even the Bali bomber.
In 2010, Williams signed with the NZRU in a bid to play for the All Blacks at the following year's Rugby World Cup. The second five-eighth made his ITM Cup debut for Canterbury later that year and was selected on the All Blacks end of season tour, becoming only the second person to represent New Zealand in union after first doing so in league.
The next season, Williams played Super Rugby for the Crusaders and was selected in the All Blacks' World Cup squad but was mainly relegated to the substitutes' bench for the team's successful campaign.
2012 brought the best rugby Williams has played, with his form at the Super 15-leading Chiefs seeing him play every minute of the All Blacks' June series against Ireland.
Many thought Williams was starting to achieve his potential in the code, finally shaking the convert tag, but that work will probably now be undone.
Indeed, Williams himself said he now considered himself a rugby player and he would prefer to watch a Super Rugby game than an NRL match.
During his time playing rugby, Williams has also boxed five times, winning all five of his heavyweight bouts to hold the title of New Zealand Professional Boxing Association heavyweight champion.
