Both lanes of the Lyttleton Tunnel have been reopened, but are restricted to residents only.
The Transport Agency says the tunnel was reopened at 3pm on Saturday.
The agency says it is important only residents use the tunnel because emergency services are relying on priority access so they can reach Lyttleton.
It says residents using the tunnel should expect significant delays as a result of periodic closures to allow fuel tankers through.
The agency says the tunnel's electrical systems and exhaust fans are operating on backup generators.