The tearful mother of one of three men who died in an Auckland manhole pleaded yesterday for the lessons of the tragedy not to be ignored.

Lynne Skeen, whose 19-year-old son Darren is believed to have heroically tried to save two unconscious workmates in the Waitangi Day tragedy, made the appeal after his employer, Aquatech New Zealand Ltd, pleaded guilty in the Auckland District Court to three health and safety breaches.

The three - Mr Skeen, fellow Aquatech employee Eddie Rihia, aged 30, and Ken Karu, 47, who worked for Stargate Services - died of hydrogen sulphide poisoning in the manhole at the corner of Halsey St and Fanshawe St while flushing out sewer lines.

It was the country's worst industrial accident for 20 years.

The court was told that Aquatech "abdicated its responsibility" to provide a safe workplace to Stargate, the company which contracted it.

Mrs Skeen said as she left court: "I hope my son's death is not in vain. I hope employers and employees learn something from his death."

She appealed during the hearing for companies to pay more than lip service to safety procedures.

Labour Department prosecutor Shona Carr said it was believed the three men could not flush out a blockage in the sewer line and Mr Rihia - whose brother, Edward Nee Nee, owns Aquatech - went down first to investigate, but was overcome. Mr Karu then went down and was also overcome.

Mr Skeen went down without rescue equipment or harness to save them, but he too passed out.

Mrs Carr said Aquatech had breached the Health and Safety in Employment Act by failing to find out the design, hazards and current state of the manhole, and had no system for passing that information to workers.

If it had, the company would have known the manhole was still under construction and it was impossible to flush out the sewer line there, she said.

Stargate had a procedure for entering confined spaces, including a check of gas levels, but this did not appear to have been followed.

Workers across the industry also did not comply with the rules, she said, although she excluded Mr Skeen from that criticism because he had worked for Aquatech for only a fortnight.

Albany-based Aquatech admitted two charges of failing to take all practicable steps to ensure the safety of its staff, and another of failing to have an effective method of identifying hazards.

Defence counsel Anne Lindsay said Aquatech and Stargate had procedures between them and Mr Nee Nee usually supervised if a worker had to enter a confined space.

The accident had caused a review of those procedures, she said.

Sentencing was adjourned until February. Stargate and Metrowater, which contracted Stargate, have denied health and safety charges and a trial is expected in May.