"We still have some of our old Ganz [trains] operating," Mr Gordon told Radio New Zealand.
Buses would be replacing trains on the Melling and Johnsonville lines this morning, he said.
The part which had raised concerns was a locking nut, held in place by a safety pin, Mr Gordon said.
"We call it the roll pin and we've been asked to check the alignment of that roll pin to make sure that it is correctly aligned for the locking nut.
"The manufacturer of this part is stationed in Germany."
The locking pins on the remaining eight trains were found to be slightly mis-aligned, Mr Gordon said.
"So we now have to have discussions with the manufacturer as to what the inspection process is from here."
The trains were still under warranty, Mr Gordon said.
• Johnsonville line: all services have been replaced by buses. They will be operating on a half hourly timetable. The first bus departed Wellington at 6.02am, and departed Johnsonville at 6.30am.
• Kapiti and Hutt Valley lines: will operate normally, however some services will have reduced seating:
• Melling Line: buses have replaced trains between Petone and Melling. Passengers should use Hutt Valley services between Petone and Wellington.
• Wairarapa line: operating normally.
Source: http://www.tranzmetro.co.nz/
