Lyttelton Tunnel has been closed so safety checks can be carried out after this morning's large aftershock centred near Lyttelton.
But NZ Transport Agency (NZTA) said today that generally state highways in Canterbury were holding up well as the region continued to experience aftershocks following Saturday's 7.1 earthquake.
NZTA Canterbury state highways operations manager Peter Connors said this morning's 5.1 aftershock may have caused cracking to the tunnel and retaining walls leading into it.
"The safety of road users is our top priority so the decision was made to close the tunnel so that a thorough inspection can be made to assess the scope of any damage," he said.
"It is likely to be later this morning before an initial assessment of any damage is available. In the meantime the tunnel will remain closed."
The tunnel links the city of Christchurch to the port of Lyttelton.
Mr Connors said a detour to Lyttelton was available via Sumner and Evans Pass, or via Dyers Rd.
He said staff, consultants and contractors were continually monitoring the state highway network in the region to ensure that it remained safe, with a particularly close watch being kept on bridges.
"The aftershocks are a challenge for those assessing the highway network as well as for those involved in repairing the damage that has been sustained," Mr Connors said.
"Most of the quake repair work in the region involves smoothing humps in road surfaces along with refilling cracks and trenches left in the wake of Saturday's quake."
Mr Connors said that one of the more challenging jobs would be repairing a 60 metre long drop out on State Highway 77 south of Glentunnel in mid Canterbury.
"An assessment will have to be made on the best option for restoring this section of highway, making it difficult to predict when it may re-open," Mr Connors said.
A 2km signposted detour is available around the affected site.
Mr Connors said the NZTA's safety assessments and repair work would continue in the region as long they were required.
NZPA