The plumber will usually clear the line, then run a camera from the cleanout toward the tank to find the break and measure its depth and distance. Because the run to a septic tank is often short, the repair may be a small excavation rather than a long trench, though decks, patios, and driveways can complicate it.
Replacing building sewer pipe needs a plumbing permit, filed online through Pembroke's Simplicity system with a Workers' Compensation Affidavit, and Building & Inspectional Services inspects the work. If the camera shows the tank inlet, the tank, or the leach field is damaged, that part of the job falls under the Board of Health and the town's own Title 5 rules.
Ask whether the plumber recommends a new cleanout near the house, which makes future clearing easier on a line that now has no outside access.