The Essential Services Commission found the company broke Victoria's energy laws on multiple occasions between March and September 2022.
Commissioner Sitesh Bhojani said 12 customers on life support and 30 non-life support customers had been impacted.
"When energy businesses fail to notify customers of planned interruptions, they not only cause inconvenience and disruption, but can also put customers, especially life support residents, at risk of harm or, at worst, death," Mr Bhojani said.
Jemena did not tell retailers that some customers required life support equipment, and did not send a particular customer information within the required time frame.
Mr Bhojani said the company needs to review its training, processes and decision-making.
"This is unacceptable for customers who rely on energy businesses to provide steady electricity supply and sufficient notice of planned outages to keep them safe," he said.
Jemena executive Shaun Reardon said the company had self-reported some matters and was treating it seriously.
"We apologise to our customers who were impacted by these planned interruptions, and recognise we failed to appropriately notify them of planned maintenance works," Mr Reardon said.
"We take seriously our responsibilities for our life support customers and the role we play in providing them with clear, timely and accurate information, and we sincerely apologise to those customers who have been impacted."
In June 2022, EnergyAustralia admitted to failing thousands of customers over life support obligations and was fined $12 million.