The Bilsdale transmitter caught fire on 10 August, leaving more than 600,000 homes across Teesside, County Durham and North Yorkshire without signals.
Arqiva, which operated the mast on the North York Moors, said the water damage was connected to third-party equipment.
It has again apologised to customers who lost services because of the fire.
Adrian Twyning, Arqiva’s chief of operations, said: “We continue to work closely with our insurers, customers and third-party companies to ensure the continued safety, security and operational resilience of all our mast sites.”
It said the investigation had taken “many months to complete” because of difficult weather conditions, the remote location and strict safety rules.
The fire-hit mast was demolished and a temporary transmitter was erected in September 2021, followed by a “more resilient” temporary structure in February 2022.
Earlier this week Arqiva said a new replacement mast would be in place before 2023.
Since the fire, the company said it had inspected 48 transmitter sites across the UK where no faults or concerns were found.
A fire which destroyed a TV and radio transmitter was caused by water getting into electrics at the site, an investigation has found
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