New ACC chairman John Judge says the corporation's liabilities have been underestimated and will increase significantly.
Mr Judge appeared before Parliament's Transport and Industrial Relations Select Committee on Thursday.
He told MPs the Accident Compensation Corporation has consistently applied optimistic calculations when working out its future costs and liabilities.
For example, ACC has made "heroic assumptions" about cost increases over and above general inflation, Mr Judge said.
He said ACC's liabilities exceed its assets, and that means it is insolvent and always has been.
Mr Judge told the committee he is as certain as he can be that liabilities at the end of June will be significantly in excess of the current $21.9 billion.
Mr Judge's appearance before the committee had been keenly awaited, after ACC Minister Nick Smith appeared unexpectedly in his place late last month.
Labour Party MPs questioned Mr Judge about the National Party's policy to investigate opening workplace coverage to competition.
Mr Judge said Dr Smith told him it was not something he needs to have on the agenda at the moment, but if the minister changes his mind and makes it a priority, it will become so.
Labour MPs also challenged Mr Judge's definitition of insolvency, saying another way to look at it is that ACC has always been able to meet its debts as they fall due.
But Mr Judge responded that he did not believe that was the correct definition for an insurance company.