An elderly man has been arrested over an attack which left a man with solvent burns to his mouth and throat.

A noxious liquid was poured into the victim's mouth while he slept rough in the entrance to the Nelson City Council building on Wednesday morning.

The arrested man, who is 79, has been charged with assault.

He's been bailed to appear in the Nelson District Court on the August 31.

Police are thanking people who came forward with information to help their inquiry.

CCTV footage also helped their investigation.