Four significant diesel spills have been recorded in Hobart's Derwent estuary in the past financial year.

The state’s Environment Protection Authority (EPA) noted the spills in its 2015-2016 annual report released this week.

The EPA does not know where they came from and says it will be difficult to investigate.

Most diesel spills were clustered at the industrial precinct at Prince of Wales Bay, but EPA director Wes Ford says it may have come from further afield.

“There could have been a diesel spill from a petrol station or a business residence, or a private residence anywhere within the stormwater catchment that flows into Prince of Wales Bay,” he said.

“Trying to locate the source and therefore the offender is very difficult if there's no clear line of evidence or somebody doesn't come forward and give us information.”

One of the spills was only reported after a resident noticed an orange discharge at the southern end of the bay.

The EPA expects there to be some impact on animal life in the bay, including seagulls.

“Birds covered in diesel will often end up drowning as a result of becoming water-logged,” Mr Ford said.

“Fish and other creatures close to the surface can ingest diesel into their gills.

“[From] the point of view of residents, it is frustrating if these sorts of spills are in our waterways, because it does impact on the amenity.”