TASMANIA'S mandatory car lock for serious drink-drivers is the only one in Australia that's applied retrospectively.
However, the state government has no plans to alter the program to have it align with other states.
None of the mandatory alcohol interlock schemes operating in South Australia, Queensland or Victoria apply to offences occurring before their introduction.
Tasmania's mandatory scheme was introduced in July for serious or repeat drink-drivers.
The decision to apply the program retrospectively but then waive the $3000 fee for those drivers has already cost taxpayers nearly $375,000 in just six months.
A further $221,652 cost has flowed from deductions given to health card holders on the program.
The government would not say what the total expected cost of the subsidy would be.
"The Tasmanian government recognises that those who committed the offence which entered them into the program prior to 31 July, 2013, didn't have the opportunity to understand the full impact of their offence and therefore couldn't plan for the financial onus that such a condition of relicensing would place on them," Infrastructure Minister David O'Byrne has said.
It is unclear why the government decided to apply the program retrospectively given it is largely based on the Victorian scheme.
The government initially forced everyone on the scheme to pay the full cost of the interlock but quietly introduced the waiver following concern from the Law Society of Tasmania and the Legislative Council.
The interlock is mandatory for drivers caught with a blood alcohol level of 0.15 or above, those with two or more drink- driving offences in a five-year period, driving under the influence of liquor or failing to provide breath or blood analysis.
The program runs for 15 months and immobilises a vehicle if the driver has alcohol in his or her body.