Yesterday, the agency pulled the certification of 34 heavy vehicle towing connections certified by Joyce, leading to four vehicles being taken off the road.
NZ Transport Agency chief executive Fergus Gammie said strong progress had been made in the bid to strengthen the agency's regulatory capabilities.
"Much more work is required.
"It's vitally important we progress this work quickly and comprehensively."
NZTA was working hard to give New Zealanders assurances that the vehicles they share the road with were certified to a high standard and were safe, Gammie said.
"While our focus is on the high priority cases identified by Meredith Connell, we have also been able to progress work on lower priority issues.
"That means a quarter of the 850 total files have now been assessed."
Gammie said the recruitment of commercial transport officers and certification officers to "significantly strengthen our enforcement capability long-term" was under way.