Backroom staff have decreased by 12% over the past three years to balance force budgets amid political pressure to keep officer numbers at record levels.
However, the Accounts Commission has found "indications police staff posts are being covered by police officers".
In addition, part-time civilian volunteers – known as special constables – "now provide a greater overall resource across the country" than in 2007/08.
Chief constables are risking breaching the European Working Time Directive, particularly when special constables go on shift after their main job.
Scottish police forces already provide the most cost-effective service in Britain at around £169.50 per head of population, compared to £191.44 in England and Wales.
Cuts in police overtime and backroom staff have driven costs down further and the Scottish Government wants to save £1.7 billion over 15 years through merging the current eight forces into a single national force.
The Accounts Commission has advised the new Police Service of Scotland to ensure police functions are "carried out by people with the right skills, knowledge and experience" amid cuts to backroom staff and increasing use of special constables.
"There are some indications police staff posts are being covered by police officers in the short term," the Commission's report on Best Value in Police Authorities and Police Forces in Scotland stated.
"But at a time of continued financial pressures there is a risk this is not an efficient and sustainable use of resources if adopted longer term."
Time worked by special constables has increased by one- quarter in the last five years, from an average of 127 hours per year to 158 hours per year.
Some forces have cut the number of special constables but increased the hours of the remaining volunteers.
Northern Constabulary halved its volunteers but doubled the hours of those it kept on, while Strathclyde Police has seen a 70% rise in hours amid a small cut in volunteers.
The commission has advised the Scottish Police Authority, which will scrutinise the activities of the new single force, to "effectively hold the Chief Constable to account" and not repeat the failings of the existing local authority police boards.