The DoE was today rapped over its oversight of its road safety advertising.
The campaign, which has included a series of hard-hitting TV ads, is conducted by agency Lyle Bailie.
The Audit Office said research on the advertising's impact is carried out by a firm appointed by the agency.
It expressed concern over a "potential conflict of interest", stating: "It is obviously important that measurement of advertising outcomes should be carried out by persons independent of the campaign design process, so that results can be seen to be independent and reliable."
The report said a memorandum agreed in 2005 gave the DoE access to all documents relating to the research.
However, it had not yet exercised this right due to "resource constraints".
Today's report said the adverts can provide income by being sold overseas.
"However, DoE does not know how the agency calculates the amounts charged for use of the material, nor the percentage of the overall proceeds that it receives," the Audit Office stated.
Lyle Bailie chief David Lyle said the 2005 memorandum on the research guaranteed independence, transparency and probity. "There are no financial links between us and the firm," he added.
Mr Lyle said the overseas sales were handled "scrupulously and above board".
The process was "very complicated", involving intellectual property rights and arrangements between the DoE and the Republic's road safety body, he added.