Police Scotland is facing an "unsustainable" financial deficit, despite receiving more money than expected in this month's budget.
The Scottish budget allocated an extra £37m to the force, almost double what it had expected.
But the Scottish Police Authority (SPA) said that still leaves an unsustainable deficit of £49m.
The concerns are detailed in a paper, which will be discussed at an SPA meeting in Stirling on Wednesday.
Reducing police officer numbers, which has been discussed, might help cut costs.
But vice-chairman David Crichton said the cash allocated for capital programmes - IT, vehicles and buildings - is not enough to provide investment to achieve efficiencies.
'Acknowledge a settlement'
He calls for new approaches to non-police work, such as officers helping people in mental health distress, to help maintain core services.
Mr Crichton paper's, published ahead of the meeting, states: "Having focused on the budget challenge at our last authority meeting, I do want to acknowledge a settlement that is better than might have been expected.
"However, we still have a responsibility to report that the policing budget remains in deficit and that this is unsustainable going forward.
"Neither the reduction in officer numbers nor the increased funding required to eliminate the deficit will be acceptable or practicable in the short to medium-term.
"The authority will therefore continue to pursue and advocate for changes that enable the deficit to be reduced."
The report states the service needs to see "faster and more effective transformational change, more robust demand and productivity analysis and detailed workforce planning".
Mr Crichton acknowledged the force has already achieved a great deal in delivering savings and efficiencies but stressed it will be a "constant process".
He added: "The anticipated capital allocation, while welcome, will not support the full scope of new investment required to achieve greater efficiencies and improved services.
"And across the wider public service system the level of failure demand which ultimately falls on police officers to resolve, for example in supporting individuals in mental health distress, will continue to draw on resources unless more creative approaches to funding and partnership working across all services are developed.
"These challenges will continue to drive much of the authority's priorities."
Last month, Nicola Sturgeon and Scottish Conservatives leader Jackson Carlaw clashed over police funding at First Minister's Questions.
The meeting will also discuss the cost of policing the UN climate change conference in Glasgow in November, which has now been estimated at £250m.