Brett Gliddon, the agency's Auckland highway manager, said the testing phase of the job was critical.
Fletcher Construction has a big role in the project as part of the alliance building it.
The twin 2.5km tunnels and 2.5km of motorway is to provide a second route through Auckland, bypassing the city centre.
The Herald reported earlier this month that the new motorway was set to open in April, most likely the weekend of April 8 and 9.
Gliddon said at the time that the only remaining works involved polishing road surfacing, line marking, signage and testing all the electrical tunnel equipment before the route can be used.
When finished the connection would provide the "missing link" between the Southwestern Motorway and SH16. The new route will provide faster more direct transport options and prevent total gridlock if an incident blocks one motorway, he said.
Existing infrastructure is feeling the strain of Auckland's burgeoning population because since 2013 Auckland has grown by 121,000 people and there are 44,000 more vehicles on Auckland roads than this time last year.
Gliddon said earlier this month that the benefits to Aucklanders would be "huge".
People who live in the north and west will have an alternative route through the city, there will be a full motorway connection to the airport and motorists can bypass the CBD on the Western Ring Route.