"Councillor Vinsen indicated to me then that he had been responsible," she said yesterday.
She said it had been a very stressful week for all the councillors and all were aware of the decision to demote Mr Vinsen before she went public.
"I can't elaborate on the detail of the breach itself, because doing that would simply broadcast the confidential detail that was involved and what council had been discussing," she said.
But she said Mr Vinsen was "remorseful" for his actions.
"He acknowledges that he made a very serious error of judgment," she said.
Mr Vinsen's admission meant there was no need for a code of conduct hearing.
He has also lost his place on the council's WWTP legal review team, set up when the council decided to file proceedings against engineering consulting company MWH Global, the company council hired to oversee the WWTP.
The plant opened in 2007 but has been dogged by problems and now faces a $24million upgrade.
Ms Main said Crs Hamish McDouall and Ray Stevens would remain on the team and she would not appoint a replacement.
She could not say whether the council's position in potential legal action had been weakened by Mr Vinsen's breach.
The audit, risk and finance committee oversees council's management and administration, investments, economic activities and other financial issues. Other specifics include setting rates, the council's 10-Year and Annual Plans and looking after the port operation.
Ms Main said Mr Vinsen had chaired the audit, risk and finance committee since October 2010 "and has been, and will continue to be, a valued and hard-working member of this council".
"However, all council members are bound by an agreed code of conduct, which requires confidentiality of information at specific times."
He would remain in other council-appointed roles as a director of Wanganui Holdings and chairman of the waste minimisation working party.
"Through these roles he has been a driver in seeking a better future for the port and the establishment of the Resource Recovery Centre in Maria Pl.
"I also expect Cr Vinsen to play an active part alongside the rest of the council in the full review of council activities currently under way. He has been particularly keen to see this work begun, seeing the potential to lower costs and to see council undertake its role in different ways," Ms Main said.
Long-serving councillor Sue Westwood, until now Mr Vinsen's deputy, has been named interim committee chairwoman.
The mayor will be committee deputy chairwoman until a scheduled review of the current council committee structure in July.Mr Vinsen had not responded to the Chronicle's messages by press time yesterday.