Giving evidence to an employment tribunal Lizzie Moore, a former personal assistant to former Welshpool Town Council clerk Robert Robinson, revealed councillors were in "absolute shock at the revelation".
The tribunal, which is considering an unfair dismissal and whistleblowing claim from former deputy town clerk at the council, Martin Bond, had earlier heard that the fire alarm system at Welshpool Town Hall had not been working for more than two years.
The town hall was closed the day after the revelation in August 2019.
Ms Moore told the tribunal she had alerted councillors on the staffing committee and the then mayor, Councillor Steve Kaye, to the problem, which led to its closure.
She said: "I remember very clearly the shock from all of the staffing committee that were there. I recall them asking how long had I known, I said I had known for a little while and they were absolutely horrified.
"I said I am just so relieved you now know."
She added: "I was the whistleblower."
"It was immediately after that meeting Steve Kaye contacted Paul McGrath (the council's operations manager) to arrange to meet him at the town hall on the Friday morning and within a couple of hours the town hall was shut. Everyone was out, all the market traders, and things were put into place to make sure it was safe and Steve was the one who instigated it and got it all sorted."
Ms Moore had earlier told the tribunal she passed regular reports to Mr Robinson from Mr McGrath on the state of the fire alarm system.
She said: "I spoke to Robert a couple of times on my own to say these are not reading well and also with him with Paul when he was saying there are things not right here."
Ms Moore said she and Mr McGrath had gone to raise the issue with the town clerk two weeks before she told councillors, and that the state of the fire alarm system had been described as "the elephant in the room".
She said: "If something happened we would never forgive ourselves, I would never forgive myself if someone was injured because I knew how bad it was. We could not cover it up.
"Paul was of a similar mind and we went to Robert and said "I am not covering any more"".
Mr McGrath had earlier said he had also raised the issue, highlighting it on weekly checklists.
He said: "He was given my checklist every week so I cannot see how he would fail to know and we definitely spoke to him about it."
Mr Robinson is due to give evidence later.
The tribunal continues.