"Further investigations will tell us just how bad the problem is."
She said the council had been aware facilities were ageing.
"It's disappointing but we know we have an ageing set of community facilities and many of them were built around the same time, so they require significant renewal at the same time, including these two facilities."
"We've got a programme underway to address that and now we'll have to look at the priority of these two sites."
Hutt City Council chief executive Jo Miller said the facilities were closed immediately as a "precautionary measure" while further inspections were undertaken.
"The health and safety of our staff and community members who use these community venues is of paramount importance, which is why we took immediate action," Miller said.
"We were planning to undertake remedial work at Petone Library later in the year to address the water-tightness of the building.
"We will now bring these works forward."
The library's first floor included a number of offices, meeting rooms and store rooms, as well as the reception.
Users of these facilities, and the Hardwick Smith Lounge, would be offered alternative venues for meetings, activities and functions, the council said.
In March, sections of Hutt Valley High School were closed after the discovery of "unsafe levels of mould", forcing some students to learn remotely for part of the week.
In 2018, Kāpiti Coast District Council closed the Waikanae Library and Service centre after air quality test results showed the presence of toxic mould.
The council advised anyone concerned about their health because of potential toxic mould exposure should see a medical professional.