Big Apple commuters were blindsided by heavy rush-hour delays Monday morning — because the MTA was doing subway repairs that were never announced.

The transit agency began work on the N, R and D line tunnels running from 36th Street in Brooklyn to 59th Street in Brooklyn — but their website didn’t indicate “planned work” on the yellow lines and declared there would be “good service.”

There wasn’t — and even MTA officials seemed clueless about why the delays were happening when they first started getting reported.

“There are no reported incidents along the lines at this time,” tweeted the New York City Transit account around 8 a.m., when responding to a commuter complaining about being stuck on an N at 45th Street.

Only about an hour later did the MTA start informing riders that it was “performing critical structural work” on the express tracks between 36th Street and 59th Street.

Commuters were fuming that they were only told about the delays, ranging from 25 minutes to over an hour, while they were experiencing them.

“How do you announce that the morning of, among all the delays??” asked @shibbyyshabamm. “This should have been posted and reported.”

“The app should’ve said ‘planned work.’ There should have been signage in the station,” tweeted Jack Szwergold. “I assure you that communication of this work between 59th St. and 36th St. was/is non-existent to abysmal. Utterly nobody knew what was happening.”

“This is not on your website. Could you please explain this major communication discrepancy?” tweeted David Mutton.

Added a rider named Joanna: “No announcements. I’ve made it 4 stops in close to 40 minutes.”

Some also wondered why the work had to begin during Monday morning rush hour.

“Why was construction scheduled and not announced during rush hour on Monday?” asked Caroline Fenn on Twitter, who said it took her 30 minutes to go one stop on a Manhattan-bound N train.

The MTA responded on Twitter, saying it would “work to make sure the delays like this morning won’t reoccur” as the work is slated to continue until December.

In a statement to Bay Ridge Councilman Justin Brannan, the MTA said the delays happened because signals weren’t properly set up to deal with service changes associated with the tunnel work.

“The MTA swore that the hot mess we experienced this morning will not happen again even though the service change on the N will remain in effect,” the statement Brannan posted on Twitter read. “It just sounds like the MTA wasn’t ready for prime-time this morning.”