He had become concerned during a visit to coastal South Otago holiday village Bull Creek at the weekend, during observation of a recreational fishing area about 4km offshore, known as Akatore Reef.
The "keen fisherman" said he was familiar with the location of the reef after "years" of fishing it, and observing other craft from land to assess conditions before launch.
"I saw this big trawler turn and head straight towards my observation point, hit the reef, then turn again and head north right up its length."
Although he could not tell if the vessel was trawling during its passage, just its presence inshore was a worry, he said.
"It's very unusual to see a vessel of that size so close to shore, and the last time it occurred a decade ago, the reef was trawled and effectively wiped out for the next three or four years."
Tautuku Fishing Club president Brett Bensemann, of Dunedin, said he was pleased recreational fishermen were remaining alert to possible incidents of this nature.
"Because this area is relatively hard to get to, it's unique in the abundance of its stocks of cod, trumpeter, grouper and the like.
"It looks like there might be a few questions to be answered here, but we need MPI to front up with the data from their monitoring systems, which will show exactly where the vessel was and what it was up to."
Recreational and commercial fishermen operated under a code of "mutual understanding", he said, and generally maintained good relations.
"We're not pointing any fingers at this stage, so we'll wait to see what the ministry comes up with before making any judgements."
Mr Walker said there would be no reason for the trawler to venture so far inshore, were it confirmed to have encroached.
"Recreational fishing is an important activity in all rural areas and we need to ensure those locations where families and fishermen can enjoy the activity are cared for.
"MPI has good GPS monitoring systems and, because this area is a vital recreational fishing area, I have asked the minister to investigate."
MPI told the ODT yesterday afternoon it was still trying to confirm whether a breach occurred.