Official papers show police might have to dip into their main budget to help prop up road policing.
The road policing budget comes from the National Land Transport Fund and was effectively frozen for ten years earlier this year.
An internal police document obtained under the Official Information Act says although baseline funding for road policing will continue, there's minimal additional funding available to absorb cost pressures, which may have to be paid for out of the main police budget.
Another document says wage growth, increased fuel costs and new initiatives will put pressure on the budget, which is capped at $310 million for the next three years.
Green Party transport spokesperson Gareth Hughes says the Transport Minister has deliberately put the police in this tight spot.
"Steven Joyce has raided the national transport pantry to fund his very expensive, uneconomical roads of national significance," he says.
Mr Hughes says Mr Joyce needs to rebalance the transport budget before police and others are forced to cut services.
Mr Joyce says it's not feasible to give the police any more money for road policing and the reality is the police have to be more efficient with the money they're given.
That's echoed by National Party leader John Key, who says the government has been working with police on their drive to be more efficient and to re-prioritise spending.
He says the state sector is recognising it has to live in tighter economic cnditions and within the government's fiscal envelope of no more than $800 million a year in new expenditure.