All front desks, apart from the one at Sunderland, will only be open during daytime hours from Monday.
The force said most people now reported crime online or via the phone, rather than in person at a police station, and the move would save money.
But Unison said the timescale was too tight and called for a consultation.
The union's regional organiser, Peter Chapman, said the plans would "inevitably" lead to redundancies.
"There may well be a need to discuss how stations provide this service in the future, however Unison believes the plans due to be implemented within the next few weeks are far too radical and the timescale far too tight," he added.
"We call on the force to abandon the current proposals and embark upon a more timely and meaningful consultation with all those involved. "
Northumbria Police said the changes would not affect the availability of officers, and the savings would be used to support operational policing and invest in greater accessibility.
There would also be further investment into the 101 contact centres.
Chief Constable Steve Ashman said: "We are still wrestling with the impact of austerity and the cuts [which] has meant that we have to make difficult decisions whilst striving to deliver the best possible service to our communities.
"We know that the public want to see police officers on the street and we are changing the way we work in order to protect this.
"We also know that residents don't want to have to go to a police station in order to report information; but rather report it in their own home and at a time that is convenient for them."
Planned overnight front desk closures at Northumbria Police stations have been condemned by the public services union as "too radical".