N.H. Rev. Stat. § 231:184
No person or corporation shall dig up any highway or public ground for the purpose of laying water or gas pipes, or other pipes or structures therein, or of repairing the same, without first obtaining the consent of the commissioner of transportation or his division engineer when the excavation is in any state-maintained highway and in all other cases the consent of the selectmen or highway agent of the town or the mayor and aldermen or street commissioner of the city; except, in cases of emergency where the public health or safety is endangered, such action may be taken as provided in RSA 236:9.
RSA 231:184
1854, 1527:1. GS 141:1. 1868, 1:40. GL 155:1. PS 82:1. PL 98 :1. RL 114:1. 1945, 188:1, part 25:1. RSA 255:2. 1981, 87:1. 1985, 402:6, I(b)(3).