Speed controls on Auckland's new electric trains have affected punctuality between Onehunga and Britomart, where the first three have run since late April.
Only 72.8 per cent of Onehunga line trains arrived within five minutes of scheduled times at final destinations in May, a report to Auckland Transport's board reveals.
That compares with a 12-month rolling average of 92.7 per cent on the line, where the first electric trains replaced old diesel clunkers on April 28.
The latest monthly figure ranks Onehunga services the least punctual of any in Auckland, a reverse of previous scores.
Initial delays were blamed on rail crew and passenger unfamiliarity with new systems, including door controls.
But the report mentions work with KiwiRail "to target short-term track speed improvements on the Onehunga line to improve both journey times and resilience to network impacts on the timetable".
It also reports a "risk review" of a new European train control system which has frustrated drivers by overriding their attempts to make the trains go closer to a 110km/h maximum speed.
A survey of 391 passengers found 97 per cent rated the new trains positively, scoring them high for appearance, comfort and ease of use.