Allan said this could have meant the upstairs of the heritage building would be undeveloped, the laneway which joins to Toitoi would not be developed and the ground floor only partially complete while the council tried to gather as much external funding as possible.
Councillor Wendy Schollum said the unfinished building is a "gaping wound" and if it is not completed it would stop the investment in the arts precinct being fully realised.
She said the entire Toitoi complex has multi-generational benefits to the community. As a child, she had performed in the Municipal Building and gone to a school ball there.
"There are countless people with those stories," she said.
Councillor Kevin Watkins said that while the additional funding is "considerable money" in context with the restoration of other historic buildings, "just" $5.5m to finish the complex is "fantastic".
He said the Toitoi complex will be "the light of Hastings" and he wants to see it open.
Councillor Simon Nixon said it was disappointing the council now had to find extra money but he "couldn't see how we cannot do it".
"If we get this far and don't finish, a big chunk gets wasted," he said.
The unbudgeted $5.5m loan will be paid back by the council over 25 years, Allan said.
When complete, the Municipal Building ground floor will include a flexible space for arts and performance, Toitoi staff offices, hospitality and retail tenants, a laneway connecting to the wider Toitoi complex, the Hastings i-Site and the Toitoi ticket office.
The first floor will be largely unchanged, with the Assembly and Shakespeare Rooms and old Mayor's Office.