Repairs and monitoring of water damage to parts of the near-new Waikato Expressway are expected to cost almost $60 million, even before a long-term fix is found.
The Transport Agency has approved $29m to add moisture sensors and waterproofing to the Ngāruawāhia section of the $2.1bn expressway opened in 2013, and $28m on the Te Rapa section opened in 2012.
The road surface has been getting rutted by water damage.
"Those sums cover the short-term repairs, as well as the cost of monitoring of those repairs and the cost of long-term repairs undertaken on part of the Ngāruawāhia section," the agency said in a statement.
"The results of our ongoing monitoring will be used to inform a long-term solution that provides the best, whole of life outcome.
"Until we've determined how moisture is entering the pavement, long-term solutions won't be developed. This also means the cost, and construction timeframe won't be known until monitoring has been completed."
The costs also covered additional works on an area of the southbound lanes of the Ngāruawāhia section, south of Kainui Bridge identified as "requiring immediate intervention".
"While suitable for the requirements of this repair, the methodology used does not represent a viable, cost-effective solution for the remainder of the Ngāruawāhia and Te Rapa sections."
Long-term fixes would probably be staged over several summer construction seasons.
NZTA and its contractors were working on how to divvy up the extra costs, the agency said.
Repairs to surface damage on the even newer Kāpiti expressway are costing $25m.
Other estimates for building new sections of the Waikato Expressway have dropped recently.
"This is mainly due to savings made through the tender process," said NZTA.
- The 5.9km Longswamp section of 5.9km, now estimated at $94m, down from $96m.
- The 15.2km Huntly section has dropped from an estimate of $409m to $384m.
- The 21.8km Hamilton section estimate is down $30m at $607m, though far above the original estimate of $430m.