The location of some highway crashes near New Plymouth have been incorrectly entered into records by the New Zealand Transport Agency (NZTA) to locations hundreds of metres away from where police say they happened.
The NZTA initially did not acknowledge the mislocations.
But it said that was due to unintentional human or computer error.
However, a road crash investigation report said the NZTA changed police records.
The location information from the police officer at a crash scene was crossed out or added to by the NZTA, or it ignored the GPS coordinates, the report said.
"Longitude and latitude ignored by NZTA," the report by an independent forensics expert said twice.
"The mislocation of crashes does not assist in the identification of crash blackspots," the report said three times.
In all, eight out of the 34 crashes since 2015 on a section of the Surf Highway, SH45, 20km southwest of New Plymouth were mislocated.
All were minor or non-injury crashes: Vehicles hit a bank or each other; in one case a truck ended up sideways across both lanes.
Six of the eight mislocated crashes occurred at a single notorious bend, with 45km/h and 55km/h speed advisory signs, where a straight section of highway enters the small winding valley at Tataraimaka heading west.
But the six had been shifted in the NZTA's Crash Analysis System, or CAS, to between 200m and 1km away along the same road.
This was revealed in the independent forensics report complete in 2018 for lawyers defending a woman against minor driving charges.
"Waka Kotahi NZ Transport Agency categorically rejects any suggestion of any deliberate inaccuracy in the recording of crash data," an agency spokesperson said.
"Crash data is only one input in a broad range of factors which are used to assess the safety of the state highway network, in order to prioritise our maintenance and improvement works."
The other two of the eight mislocated crashes only come to light since RNZ began making inquiries.
The NZTA then checked up on 23 other highway crashes at Tataraimaka, and found two in the wrong place.
Asked if this suggested up to 10 per cent - two out of 23 - of crash location records nationwide could be wrong, an NZTA spokesperson said no.
However, last night the NZTA came back to RNZ, saying it was "still assessing" the extent of the mislocations.
"We believe the issue is limited to the geospatial coordinates associated with mapping of some crashes in the old CAS system."
The system was updated last year.
Bend's crash history
The crash history of the worst of Tataraimaka's bends became a key factor in the defence of a woman charged with careless use after two vans collided in mid-2018.
The forensics expert, Dr Tim Stevenson, declined to speak to RNZ.
His December 2018 report noted that a crash in early 2017 "has been mislocated by the NZTA, so that it does not come up as a result when searching for crashes that occurred on the same bend".
"Longitude and latitude ignored by NZTA, '390 m south of Timaru Br' added," his report said.
He found five other such crashes.
"Crash on same corner, but relocated by NZTA with longitude and latitude ignored, '500m south of Weld Road' crossed out, and '140 m south of Timaru Br' added."
Five of the six mislocated crashes were on a wet road.
By checking the NZTA's own skid data for the bend, Stevenson concluded the slippery, "defective" road surface was mostly to blame, contradicting police, who had prosecuted the driver.
Start, not end of crash recorded - NZTA
RNZ contacted several road crash investigators, who expressed surprise the NZTA would mislocate crashes.
They declined to be interviewed or identified, though one said it was "very serious" as they relied on the location data for their investigations.
Initially, the NZTA did not acknowledge the mislocations.
It said its locations were different from the police because its nationwide approach was to record the "start of a crash", not the end.
This could help determine the cause, it said.
"So a single point won't necessarily reflect a scene that could potentially be hundreds of metres in length.
"On rare occasions, if we have exhausted all possible avenues of investigation, we may use the end point of the crash."
Occasionally, differences were due to the NZTA fixing up "discrepancies" about locations in police records, it added.
Some days later, however, the NZTA admitted it had put crashes in the wrong place.
"While we make every effort to ensure that the information captured by CAS is accurate, incorrect data can be unintentionally recorded, either as the result of human error or system error."
Three mislocations were due to human error, five to computer error in the previous crash analysis system.
"The crash locations will be corrected in CAS for all of these crashes.
"A new CAS system was introduced in February 2019, which includes a series of automated business rules and functions specifically designed to help prevent the unintentional input of incorrect information as has occurred in these instances."
More work was going on to increase the overall quality of crash data, it said.
The NZTA would not be interviewed on tape.
Roadworks, but crashes continue
The NZTA did roadworks at the worst Tataraimaka bend in June 2018 to improve grip.
However, there have been at least three more crashes there since then.
Two local people told RNZ they were very used to the crashes.
Police have previously spoken about how numerous collisions were.
The stretch of SH45 rates as a "low priority" for upgrading in the NZTA's road risk assessment system, Mega Maps.
It has a lower priority than the straighter stretch of SH45 west of Tataraimaka valley.
Improving roads is one of five focuses of the "Road to Zero" safety strategy adopted by the Government for the next decade.
It aims to cut deaths and serious injuries by 40 per cent.
The NZTA did more highway maintenance last year than ever before, under an improved contracting model it introduced in 2013, it said.
The Network Outcomes Contracts "ensure that the right work is done in the right place, and at the right time".
"Having a finite budget does mean that we have to continuously prioritise and make trade-offs to manage a state highway network which has grown significantly, while the number of vehicles using the network has increased," it said.
It had robust ways of identifying high-risk roads.
"This includes an annual physical examination of every kilometre ... [for] skid resistance, texture, roughness, rutting ..."
The quality of highways is most controversial on the Napier-Taupō Rd, SH5, where eight people have died since December.
The Taranaki Surf Highway at the mislocations area has had relatively few serious or fatal crashes.
The NZTA did not go to minor crashes, but at every fatal collision it checks road conditions, the surface, signs and markings "at, or as near as possible to, the time of the crash".
RNZ looked through the police records of all 37 crashes reported in the area from 2015-19.
In about half, it was difficult to match them with NZTA crash locations, however, this is at least partly due to the public face of CAS not giving a specific date of a crash - only the year it occurred - for privacy reasons.