Hutt City Council was told the roof of Naenae learners swimming pool was a high earthquake risk two months before the pool was closed, an engineer's report shows.
Beca Engineering wrote to Hutt City Council (HCC) on 26 September saying the roof was just 20 percent of the New Building Standard.
However the pool wasn't shut until 1 December.
A building with an earthquake rating of less than 20 percent is about 25 times more likely to fail during an earthquake and is deemed a "very high risk".
Anything less than 65 percent is an earthquake-risk building.
The council had commissioned Beca to undertake the assessment as part of a major refurbishment planned for 2020.
HCC general manager of City and Community Services Matt Reid said the learner's pool could legally remain open but the council had taken a cautious approach to close it on 1 December after it received the full report in November.
He said the September report was concerning but did not warrant an immediate pool closure.
"Clearly there was cause for concern... this was an initial assessment and that assessment did prompt us to do a more detailed one. There was concern, but at that point there wasn't enough to say 'we need to take immediate action with this learner's pool'."
Mr Reid said the council was comfortable in the decision to keep the pool open.
The report referred to an initial assessment report of the roof in which it was confirmed the roof of the complex had no bracing and was in the order of 20 percent of the New Building Standard.
"There are no details in the design drawings of a structural roof bracing system," the report said.
The original design notes from 1977 state that the curved shape of the roof and the foam ceiling insulation should act as roof bracing.
"Beca do not agree with this approach. In our opinion the [insulation] is not considered suitable to act as a dependable roof-bracing system to carry seismic loads."
The November report also cautioned of a large suspended service duct and "various suspended items" that could present a life-safety risk.
"Heavier items, such as the suspended services duct may present a life-safety risk to occupants if not properly restrained."
The report did not confirm if the duct was restrained properly as it was out of the engineer's scope to report back on.
Mr Reid said the duct had been checked in the last 12 months as part of the council's annual maintenance checks and he was confident it was restrained properly.
He said a thorough investigation, which included assessing the duct, was done after the November 2016 Kaikoura earthquake.
Similar ducts fell at the Keith Spry Pool in Johnsonville after the Kaikoura earthquake.
Lyn Bareta, a trustee for community advocacy group Team Naenae Trust, said it was concerning that safety issues had been kept in the dark, but wanted the focus to be on moving forward.
She wanted funding for the pool fast-tracked so it could be re-opened as soon as possible.
Mr Reid said it was unclear at this stage whether seismic strengthening would happen at the same time as a planned refurbishment in 2020, or sooner and said the council was seeking expert advice on this.
The main pool is currently undergoing a detailed engineer's assessment.