Police are singing the praises of a new piece of search technology - but it hasn't managed to help solve a 25-year-old missing person case.

Joe Freiman, a 64-year-old farmer, went missing during a hunting trip on Stewart Island in 1991 and was never found.

Search and Rescue over the weekend resumed the search for Mr Freiman in order to test a new radio transmitting tool they've been trialling.

About 80 police and volunteers from the Southern District made use of the SARTrack system as they searched the area on foot for two days with no air support.

The system is designed to allow a controller to get real-time updates on where all search teams are, while tracking overall progress and making notes.

But while Senior Sergeant Brian Benn said the tool had given excellent feedback and would be used more often in searches of remote areas, no new leads had been found in Mr Freiman's case.

"These cold cases are excellent material to base SAR exercises on as they highlight how little information we often have to go on during these operations," he said.

"They also provide an opportunity to take another look at these historic cases with fresh eyes and the benefit of modern technology and search methods."

But he said police did turn up an interesting item that proved the value of the technology: a small pocket knife that had been lost in the search area a few days before.

"It's a testament to how thorough our search was when throughout a 10 square kilometre search area, that small pocket knife was found within the bush," Snr Sgt Benn said.

"Considering the difficult terrain in the area, where walking just five metres is a challenge, this exercise was a model of best practice."

Mr Freiman vanished after leaving his party at the Mid Big Glory Hunter Camp to go hunting along for a few hours on May 30, 1991 and failed to return in the evening.