In a brief of evidence which was read to the court by MBIE prosecutor Shona Carr, the worker said his job was to replace an air valve which he accessed from a shallow chamber on Mays Rd, Onehunga.
He described using a blow torch to remove the valve because it had corroded onto the pipe.
When he was three-quarters through cutting off the valve he heard a sudden rush of air out of the pipe. "It then seemed to get suddenly sucked back into the pipe. I didn't think anything of it."
The worker continued working, and a short time afterwards heard the sound of a fire engine.
He then received a text message from his boss saying: "Something has happened on the head job, are you okay?"
A worker from HEB Construction, another firm contracted to Watercare, then arrived on site and told him there had been an explosion.
The CPL worker said he knew there was another job elsewhere on the pipe, but didn't know what it was, and didn't know anyone was going inside the pipe.
"Our job was just a normal valve replacement job which we did all the time."
Under questioning, he said it wasn't his practice to use gas monitoring in jobs of this type, as the shallow air valve chamber was not considered a confined space.
Lawyer for CPL, Paul White, told the court the company would have pleaded guilty if it thought there was any way it could have prevented the incident.
"There was nothing in that circumstance that would have warranted it acting differently and nothing that could have been done to avoid the harm that occurred or even avoid the possibility of harm."
Mr White said the chamber where work was being carried out could not be defined as a confined space under the applicable safety standard, so there was no requirement for gas monitoring.
"This was performing hot work in a shallow chamber in the middle of the road in Onehunga with houses all around, where no reasonable person would consider there is the possibility of explosive gas being present."
Earlier, Mrs Carr told the court the worker's LPG-powered gas torch probably provided the ignition for the fatal explosion.
It ignited natural gas from the Vector gas distribution network which had been drawn into a water main through its air valves while being drained.
MBIE alleges CPL failed to identify the hazard of explosive gas, and failed to monitor the atmosphere in the work space for explosive gas.
The company also failed to follow Watercare's procedures relating to working in restricted spaces, which would have required gas monitoring, MBIE alleged.
CPL faces two charges, failing to take all practicable steps to ensure its employees were not harmed, and that it failed to take all practicable steps to ensure its employees' actions did not cause harm to others.
Watercare Services Ltd earlier today pleaded guilty to two charges, while a third was withdrawn by MBIE.
One of the charges related to failing to take all practicable steps to ensure the safety of its employees, and the other to failing to take all practicable steps to ensure the safety of its contractor's employees.
The withdrawn charge related to failing to ensure the safety of CPL employees.
Watercare will be sentenced next week.
