A decaying historic kindergarten building in Victoria Park is about to be restored.

For more than two decades, the Campbell Free Kindergarten opposite the Victoria Park Markets in Victoria St has been empty. Now its roof has fallen in and the building is in a poor state.

But the Transport Agency and Auckland City Council have reached a tentative agreement over it.

The agency needed a structure near the new Victoria Park Tunnel for its back-up generator equipment, operated in emergencies during a power failure.

Auckland City wanted the kindergarten restored, after it was vacated more than 20 years ago.

So the two organisations struck a deal which will see the agency build a small lean-to on the side for its diesel generators. It will put other equipment into the space on the building's first floor. But most of the downstairs area will be turned into a public meeting space, able to accommodate up to 100 people.

Graeme Easte, an Auckland City and Western Bays councillor, has led the effort to broker the deal and he said the outcome was highly successful for all parties.

The exterior of the building would be restored, he said, and the lean-to would be built on its western side.

"The building's in a terrible state right now with taggers and trespassers coming in there. The roof over the portico looks like it will collapse and the stairs inside don't exist much any more. But the brick structure is sound," Mr Easte said.

Tommy Parker, the agency's Auckland and Northland state highways manager, said the tunnel project would leave a legacy by restoring the kindergarten.

Darren Utting, Victoria Park tunnel project stakeholder manager, said his organisation was pleased with the decision.

"It's a great example of collaboration between two regulators."