Repairs on the $630 million Kāpiti Expressway are ongoing as the road continues to spring leaks.
Last year, The New Zealand Transport Agency (NZTA) revealed that 14km of the 18km road needed to be resealed, after water and discolouration began appearing on the surface.
NZTA said it would work to fix the problem, but so far only 2km have been repaired.
Agency spokesperson Chris Hunt said repair methods had been trialled on the remaining sections - but all of those had failed.
"The M2PP Alliance needed to develop a methodology to ensure the seal repairs would be lasting and unlikely to discolour again," he said.
"During this development, some areas were resurfaced to avoid further deterioration in the pavement. While some of these discoloured again, these patches allowed us to work out the best treatment for the full remedial repairs which we have committed to."
He said trialling new repair options was a lengthy process, which was why only 2km had been fixed.
"We had to take time to develop the right methodology through trials and be sure that the fix would be lasting.
"The work is weather dependent and we have also needed [to] balance our resources with the ... surfacing work which has been a key priority."
Despite concerns, Mr Hunt said the expressway was safe to drive on.