The crew of the Argosy cargo plane aborted the approach to their destination of Wellington Airport because the left hand undercarriage failed to lock down. It was decided to divert to Blenheim-Woodbourne Airport (BHE) where the SAFE Air engineering base was located. During landing rollout the left main undercarriage collapsed and the airplane veered off the side of runway 25. The failure of the left main undercarriage to fully extend was caused by the seizure of the piston rod within the undercarriage jack assembly. The piston rod and the spring housing through which it passed had become scored, eventually resulting in the seizure. The piston rod had been installed, new, in the jack assembly some seven months earlier and had completed 787 landings. Further investigation found that the synthetic wiper ring had been installed incorrectly and would allow foreign material to find its way into the spring housing when the undercarriage was retracted. Additionally, hydraulic components on the undercarriage may have become contaminated during cleaning in an inadequately filtered cleaning booth, which was also used for general cleaning involving the use of abrasive pads.