They were exposed to fumes from petrol-driven equipment in the tunnel, which did not have a ventilation system.
Gas monitor alarms were set off, but a group of six contractors, who were about 3km into the tunnel, continued working until they started suffering the effects of the gas.
After the incident, a contractor from Concrete Solutions said the workers became ill and started dry retching after inhaling fumes and dust.
The workers left through the farthest exit, which exposed them to carbon monoxide levels well over the short-term exposure limit for more than an hour and a half.
A KiwiRail worker found them lying on the ground outside the tunnel, where they were trying to regain their breath.
KiwiRail said its recovery procedures for an event of that level were followed on the day, including rest, fresh air, food and drink.
It had now changed its procedures to include medical tests.
Former New York firefighter Mike Ross, who has attended many carbon monoxide poisoning incidents, including deaths, said he was astonished the workers were not given immediate medical help.
"Carbon monoxide exposure that's severe enough to cause the symptoms that were reported - light-headedness, dizziness, migraines - that should absolutely provoke an immediate medical response.
"Those guys should have been having their blood gases monitored, they should certainly have been given oxygen immediately - that's the number one first aid for carbon monoxide."
Mr Ross said carbon monoxide exposure did not have life-long repercussions, but put extra strains on the body.
Rail and Maritime Transport Union general secretary Wayne Butson said the workers were unprepared for the hazards. The incident had raised concerns about New Zealand's health and safety record, which was a "diabolical disgrace".
The incident was reported to the NZ Transport Agency, which exercised its discretion not to refer it to the Transport Accident Investigation Commission, which investigates serious rail incidents.
KiwiRail said it took safety seriously and the recommendations from its internal investigation were immediately implemented.
It accepted the evacuation should have taken place sooner, and said it had changed procedures to ensure all staff working in the tunnel had gas awareness training.
KiwiRail said carbon monoxide levels on the day had barely exceeded a quarter of the ceiling at which gas masks were compulsory.
It disputed there was not enough safety equipment, saying all staff had gas masks.
Neither Labour Minister Simon Bridges nor Transport Minister Gerry Brownlee would comment on the incident.
A Ministry of Business, Innovation and Employment official said it appeared the ministry was not notified of the incident until yesterday.
The ministry had requested the Kiwi Rail and Rail & Maritime Trade Union reports.
It would review them and then take further action as appropriate.
- Additional reporting Kate Shuttleworth
