The New Zealand Transport Agency (NZTA) provides a great online resource to help you plan your trip and get up-to-date information on the traffic conditions at both a national and regional level - including live updates, regional maps and webcams. 

View a list of major road closures, road works and any other activity that may affect your travel plans, as well as a report of the current traffic conditions and congestion in the major cities of Auckland, Wellington & Christchurch.

This includes detailed information regarding:

1. the condition of State Highway 1 between Picton and Christchurch, following extensive damage to the road caused by the November 2016 earthquake.  The road is fully open again (from March 2018), but there are still multiple road work sites with potential delays.
2. the status of State Highway 3 (Manawatu Gorge) linking State Highway 1 & 2 between the Manawatu and Hawkes Bay regions.  This road has been closed since a major slip occurred in April 2017.  An alternative route is detailed.
   
   

Be sure to check out the Traffic and Travel information page before you start your journey.