The 47-year-old victim, Sean Patrick Johnson, of Dervock, died in the incident, which took place in October 2006 at a NI Water waste treatment plant in Portrush.
An inquest jury in Coleraine said that while the exact cause of the accident was unknown due to the fact there had been no direct witnesses, there were "defects in the system of working which contributed to the accident".
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The jury ruled Mr Johnston was "one man doing a three man job" and said there were "unclear instructions" regarding the work he was involved in.
The jury also cited "inadequate supervision" at the West Bay pumping station, while ruling that Mr Johnson being a non-swimmer contributed to the circumstances of his death.
Mr Johnston had started working as a telescopic handler on the construction site just four days before his death.
He was employed by John Graham Ltd to help build a waste treatment plant for the Water Service, with the aim of improving the water quality at bathing beaches.
John Graham Ltd has already been fined several thousand pounds at Antrim Crown Court after pleading guilty to charges brought against them by the Health & Safety Executive in connection with the case.
Mr Johnson plunged more than 10 feet into the water as he worked to move steel mesh which formed a temporary covering on top of the six-metre well, which was around half full with water.
(NS/GK)
