The transport agency is refusing to release a briefing on two public-private partnerships used to build expensive sections of state highway.

Waka Kotahi prepared a detailed status update on both the Pūhoi-to-Warkworth and Transmission Gully motorways in December.

While both roads have been open for months or more, Transmission Gully remains technically unfinished and beset by legal wrangling.

Pūhoi, though it missed deadlines as it struggled with landslides, appears on track for a final close-out in December.

The existence of the Waka Kotahi update was disclosed in a Treasury note, but Waka Kotahi rejected RNZ's Official Information Act request to release it on the grounds of commercial sensitivity and legal privilege.

The Infrastructure Commission, in a long overdue independent review earlier this month, found the public-private partnership (PPP) was not the root cause of Transmission Gully's time and cost overruns.

The final bills have yet to land for either motorway due to contractors' claims still being worked through.

Nominally, the 27km-long Transmission Gully cost about $1.25 billion, and the 18km Pūhoi motorway possibly about $900m - but the latter has been hit by claims of more than $200m, related to Covid delays.

The latest progress report on Pūhoi, released under the Official Information Act, had two full pages about "claims and determinations" completely blanked out. It listed 24 outstanding issues and five "high risks" to completely finishing the project, but said there had been good progress on them.

The Pūhoi-to-Warkworth motorway is likely to cost more than $1 billion. Photo: Screenshot / RNZ

Forty-two design changes had been finalised by April, and only three were still being worked on.

The Pūhoi PPP consortium missed a goal of opening the road in August 2022 by several months, but it had made "excellent progress" on fixing non-compliant items as full works completion neared in April, the report said.

Final "close-out" tests were forecast for December 2024.

The Pūhoi motorway goes through patches of unstable land. RNZ earlier revealed that a large, slow-moving slip had cracked concrete barriers and damaged the road shoulder at a northern section, around the time the popular motorway opened in mid-2023.

Waka Kotahi confirmed at the time there was deep-seated movement.

It now said this site was fully repaired by September 2023 and was causing no concern now.

"Regular monitoring reports are produced by the contractor as part of this exercise."

At one point there was a slip on surplus land that needed fixing, the progress report said.

This story was written by Phil Pennington, and originally posted on RNZ News