She signed an ACC form every year until 2012 stating she was not married or in a de facto relationship.
However, an ACC investigation discovered her new partner moved into her home in 1992.
They had a child together later that year, and the man became joint owner of the property in 1996.
The couple acquired a further 10 properties over the years.
ACC's investigation in 2013 determined their relationship was "in the nature of marriage", and weekly compensation to the woman should have stopped in 2004.
Her payments stopped following the investigation.
She admitted in an interview with ACC that she lived with the man and that they had a child together.
But she denied their relationship was de facto or in the nature of marriage.
ACC calculated she was overpaid more than $373,000 between 2004 and 2013.
The woman was charged with two counts of using a document for pecuniary advantage, and seven counts of dishonestly using a document.
Judge David Ruth said during sentencing on Wednesday there were several aggravating factors, including the amount, the repeat offending and the breach of trust.
"This type of fraud is easy enough to perpetrate, and difficult enough to discover," he said.
"The system of social welfare relies very much on the honesty and integrity of recipients."
Despite maintaining her innocence, the woman had "paradoxically" offered to repay the full amount to ACC, Judge Ruth said.
Most of the money had been raised by her son, and there was $75,000 in a term deposit account that would become available in a month, he said.
She was sentenced to two years' imprisonment, which was converted to 12 months' home detention.
She was also sentenced to 150 hours' community work, and to repay $373,000 within 40 days.
If the woman had not paid the full amount by May 25, the court would apply to review the sentence and the only option would be prison, Judge Ruth said.
Her name was suppressed until that date.
Her lawyer said the woman was likely to appeal the decision.