The aged care watchdog knew for four days a Melbourne nursing home staff member had tested positive to coronavirus before telling the federal government.
St Basil's Homes for the Aged in Fawkner has been linked to at least 20 coronavirus deaths.
The nursing home came under fire at a Senate inquiry last week when federal health officials said they weren't made aware of the first case until July 14, five days after the nursing home knew.
But in a letter to committee chair Katy Gallagher, Aged Care Quality and Safety Commissioner Janet Anderson revealed the watchdog knew on July 10.
Ms Anderson said an official from St Basil's told the aged care commission over the phone about the staff member's case.
She said Victoria's public health unit was also notified.
"This information was escalated internally and recorded in the commission's daily COVID-19 confirmed case tracker," Ms Anderson said.
"The commission did not escalate the matter externally at the time because the St Basil's representative had confirmed in the interview that they had advised the PHU of the outbreak."
Prime Minister Scott Morrison said he was made aware about the updated timeline on Sunday evening.
"I am concerned about that breakdown in the communications," he told reporters in Canberra on Monday.
"There had been a breakdown in that communication and that's not good. I understand the processes that led to that have been changed and I'm undertaking further enquiries into this."
Mr Morrison has stressed the watchdog operates independently to government.
Labor's aged care spokeswoman Julie Collins said the communication failure led to a potentially deadly delay.
"Time and time again the Morrison government has been warned about information sharing failures between its regulator and the Department of Health but has not acted," she said.
"We are now seeing the tragic and avoidable consequences of not properly heeding these warnings."
Australian Associated Press