Gov. Kathy Hochul vowed Monday to get to the bottom of an “unprecedented system breakdown” that caused a five-hour MTA service disruption Sunday, explaining that the “unacceptable” situation was caused by a brief Con Edison power failure and a subsequent power surge.

About 8:25 p.m. Sunday, ConEd lost a feeder “for a short period of time,” which caused a “voltage dip” in the five boroughs, as two power plants went offline, the governor said at a press conference Monday morning.

“It was a momentary outage that did go to the backup system. But when it tried to go back to normal, there was a surge — an unprecedented surge — that resulted in the subway losing signalization and communication ability,” Hochul said.

“There was a voltage dip, and when they tried to get back on, the backup system worked, but all of a sudden, there was an unprecedented surge at the time they were going back online, and it caused the signalization and communication shutdown.”

A total of 83 trains along the 1, 2, 3, 4, 5, 6 and 7 lines, as well as the L train, came to a temporary halt, according to the governor. “Most concerning” among them were the five trains that came to a standstill in the tunnels between stops, leading to evacuations of hundreds of riders, said Hochul.

The MTA’s subway regular service was restored about 1:30 a.m. Monday, Hochul said, explaining that bringing trains back to standard schedules was delayed past midnight by train riders self-evacuating, which forced firefighters to search the tunnels for riders.

“This was a very dangerous situation that did delay” the return to normal for the Big Apple’s underground public transit system, she said.

Two of the evacuations were carried out in an “orderly” fashion, while during two others, stranded commuters performed a “self-evacuation,” where they left the subway cars on their own, the governor said.

On Sunday night, hundreds were evacuated when two trains got stuck in the Harlem River tube.

A southbound 2 train and northbound 3 train were traveling through the tunnel about 10:40 p.m. when a transit power outage disrupted service on several lines. About 150 people were evacuated from the southbound 2 train, and 250 were evacuated from the northbound 2 train, sources previously said.

“Last night, one-half of the New York City subway system experienced an unprecedented system breakdown,” said Hochul. “This is a scary situation — something we don’t want New Yorkers to have to experience again.”

The confluence of events has never happened before, to our knowledge.”

Hochul, the newly minted governor, said the service suspension was “unacceptable.”

“If you’re one of those riders, people relying on safe transport, the system failed you,” she said. “The MTA is the lifeblood of this city, and a disruption of this magnitude can be catastrophic.”

Hochul stressed that there is no foul play suspected in the power problems that caused the delays and that they were unrelated to the manhole fire in Long Island City.

The governor announced that she’s “immediately directing a review” to find the “root cause” of the five-hour subway stoppage.

“We need to know why the system broke down,” she said outside the Bowling Green 4 and 5 station. “We will learn lessons from this, and make sure it doesn’t happen again.”

During his daily press briefing, Mayor Bill de Blasio said he spoke to Hochul about the snafu, and demanded answers  from the MTA and Con Ed about what transpired in order to prevent a similar incident.  

“The good news is we had a temporary situation, but we need to know more about it,” he said. “There were not, thankfully, extensive home outages.”

“But at the same time, a lot of subway riders were really inconvenienced, and put into a really tough situation,” the mayor added. “We’ve got to figure out why this happened, and make sure it doesn’t happen again.”

The transit workers union hailed their members’ performance during Sunday’s outage, claiming the incident proves the prevailing two workers per train arrangement is necessary.

“Transit workers did a great job during the emergency, demonstrating once again why you need a fully staffed system, and that includes having both a Conductor and a Train Operator on the train,” said TWU local President Tony Utano in a statement. “You need all hands on deck during emergencies and we see once again why OPTO (One-Person-Train-Operation) would be a disaster.”

“Conductors walked through the cars of stalled trains – urging riders to remain calm, and providing them the latest information they had,” he added. “There will always be emergencies to contend with in an underground system: flooding, power outages or surges, fires, criminal activity and stalled trains requiring evacuation. You work to prevent as many as you can but you can’t prevent them all.”

Additional reporting by Julia Marsh and David Meyer