The majority of Tasmania will lose guaranteed next-day postal service for interstate deliveries from April 8.
Launceston's Australia Post outlets are some of many in the state that will be affected by the alterations.
Under the changes, only 22 postcodes - all in the greater Hobart region - will be guaranteed next-day delivery for interstate Express Post parcels.
At present, all Tasmanian postcodes are listed by Australia Post as being able to utilise the service.
Intrastate Express Post next-day guarantees will not be affected.
A statement from an Australia Post spokeswoman said air freight arrangements between Tasmania and the mainland were to blame for the changes.
``Express Post products will continue to be available at all outlets and they will continue to provide a faster service than regular parcel post delivery,'' the spokeswoman said.
``However, the next business day guarantee will not apply to Express Post products sent between Launceston and interstate locations.''
A Launceston Australia Post outlet owner said the decision would have a massive impact on customers.
``We would send and sell at least 1000 Express Post parcels a week - and we are just one outlet,'' the owner said.
``This is going to have a massive impact on customers . . . legal firms sending documents, real estate firms sending documents . . . everyone.''
Bass Labor MHR Geoff Lyons said he was not aware of the changes.
``If this is the case I'll be fighting like hell to keep [the service],'' Mr Lyons said.
``I can't believe that's going on. I'll be telling the minister, in no uncertain terms, that Tasmania is part of Australia and we deserve this service.''
Launceston alderman Rob Soward also vowed to fight to retain the service.
``I'll be putting a motion to the council on [February 25] that we fight to overturn this decision,'' Alderman Soward said.
``This will have massive ramifications for everyone.''
Australia Post said there would be no impact on staffing levels and more information would be available about the changes soon.