Butson was relieved the three staff members, who are also union members, weren't seriously injured and said support would be offered to them and their colleagues.
He said there was a need for ongoing focus on health and safety following the accident.
The accident has caused disturbances to the Eastern Line trains.
Auckland Transport said all services are now cancelled including between Manukau and Otahuhu.
AT have organised replacement buses that will be running all day and are accepting rail tickets and AT Hop cards.
Gordon said it was too early to determine the cause and a full investigation would be undertaken.
He said staff were working to reopen the line as soon as possible.