The trucking fleet is believed to be dodging more than $50 million in road user charges.
Documents obtained by Radio New Zealand also show the number of investigations into such cases has dropped dramatically.
The Transport Ministry estimates truckies failed to pay almost $51 million in road user charges in the 2010/2011 financial year, a rise of more than $10 million on the year before.
The unpaid amount is equivalent to nearly 5% of the more than $1 billion the levy generates.
The Transport Agency says while it can not be sure of the exact reason for the estimated increase this is an area where its performance needs to improve.
The charge helps pay for the maintenance and building of roads and the Automobile Association says all road users lose out when people don't pay their fair share.
Both the Association and the Road Transport Forum which represents the trucking industry say evasion could be eliminated if the road user charge system was dumped in favour of a diesel tax which they say would be impossible to avoid and cheaper to run.
Labour Party transport spokesperson Phil Twyford says it is disappointing trucking operators are gaming the system.
Figures also show the agency is conducting fewer investigations into evasion.
In the 2008-9 financial year the Transport Agency carried out 256 investigations, collecting some $4 million in unpaid charges.
That number fell to 79 investigations in the 2010/2011 financial year which resulted in $2.2 million being recovered.
But in that year, the average settlement was much higher than normal at almost $28,000, and every individual or company investigated was found to have evaded paying road user charges.
In the past three years it has cost between $14 million and $17 million a year to administer RUC collection, investigation and enforcement.
The Transport Agency says a simplified road user charges system coming into force in August will reduce the opportunities to evade paying road user charges.
