A Second Avenue Subway subcontractor filled its job site with cheap apprentices, then billed at a much steeper rate, a former worker charges.

“There’s a common saying down there — the MTA is the ATM. You get money if you’re standing,” disgusted whistleblower Alexander Maack told The Post about contractors’ attitudes toward big projects. “Just show up for the day and you make the boss a profit.”

The MTA Inspector General’s Office confirmed that it is investigating the allegation.

Maack, 42, said he began working for Celtic Sheet Metal — which subcontracts for city contractors E.E. Cruz and Tully on the $4.45 billion Second Avenue Subway project — at the 86th Street site in November as part of an apprenticeship.

Maack said a foreman pressured him to present himself as a mechanic to an outside construction manager — even though he was still a student at the Nicholas Maldarelli Training Center.

A first-year apprentice makes $34.19 an hour, but a mechanic makes a staggering $94.11 an hour — a difference of $60 an hour per worker. He alleges that his company billed the contractors E.E. Cruz and Tully the higher rate.

“An apprentice is someone who is just beginning out,” Maack explained. “They’re supposed to gain a skill set by working with their hands and learning a craft. A mechanic is at the highest level. They know much more than you.”

Maack, who received an apprentice paycheck from Celtic Sheet Metal, said he was able to avoid lying by telling the construction manager to ask the foreman what his title was.

But the situation escalated when his supervisor tried to bully him to lie again to a Department of Labor rep in January, he said.

“I went to pull out my paycheck. When I pulled it out, my foreman freaked out and said, ‘He’s only playing’ — and told me to go upstairs immediately,” Maack said.

Maack said he was banished for the rest of the day to a shanty where workers change their clothes — and transferred to a non-MTA site the next day.

He worked at Brookfield Place downtown until the end of March.

“Day one on a new job, you have to introduce yourself to the foreman,” Maack said.

The higher-up immediately told him, “ ‘Nobody wants to hear your s- –,’ and, “Keep your nose down and your mouth shut. Start talking and you’ll be on your a–, out of the job,’ ” Maack said.

Maack fumed for weeks — and said he was pulled off the site in March when he angrily told Celtic he would tell authorities about them.

He reported the subcontractor to the MTA inspector general last week.

Spokesman Michael Boxer confirmed that his agency was looking into the matter.

The MTA said it was assisting the probe.

“These allegations would not impact us financially because we pay the general contractor, E.E. Cruz and Tully, a lump sum,” the MTA said in a statement. “However, we want to be sure that the quality of the work promised is what we’re getting.”

Straphangers’ advocate Andrew Albert said if the contractor is being cheated by the subcontractor, it could harm the work being done.

“The risk to the MTA and its riders is that because the contractor is not getting their money’s worth from the subcontractor, somewhere in the process somebody will try and make it up with shoddy materials, or we will end up with a job that’s not up to industry standards,” said Albert, of the Transit Riders Council.

“Another possibility is that the job ends up costing the MTA more money, especially if the contractor comes back to the MTA asking for additional appropriation.”

Celtic Sheet Metal said they will cooperate with the MTA I.G.

“These are false allegations,” said their lawyer, Gerard Brady. “We reject all of these claims.”

Additional reporting by Natalie Musumesci