A Ministry of Transport document reveals the road policing budget is under intense pressure from wage costs.
Road policing, which is paid for through the national land transport fund, has effectively had its funding frozen for the next decade.
The document released under the Official Information Act shows about 80% of the current budget of $300 million a year goes on wage costs.
In recent years, this has been the most significant factor in spending increases.
In a Transport Agency briefing to the Minister, it says there will be greater transparency and scrutiny of value for money in the next three years and police may have to consider internal trade-offs to keep within budget.
The Transport Ministry recommended a funding range that included a 3% annual wage increase, but the Government rejected this.
The Green Party's transport spokesperson, Julie Anne Genter, says this was a mistake.
"This is the cost of the Government's unbalanced transport policy. Funding has been frozen for road policing and pretty much every other category of transport expenditure except state highways."
Police have declined to comment on the situation.