Adrian Britton has made a fresh appeal to catch the person who aimed a shotgun at mother-of-three Dorothy Britton and shot her once in the head at her Airlie Beach home in March 1996.

He believes his mother probably knew her killer because they would have had to make it past her Rottweiler, which was extremely protective of both Dorothy and her home.

Cold case detectives, who've been following leads in South Australia and north Queensland, believe the killer or killers knew Dorothy's property, her dog, and her movements.

Mr Britton says the crime has spawned suspicion that's fractured his family for more than two decades.

"There's still suspicion because we haven't got closure," he told reporters in Townsville on Monday.

"Even from the get-go, there was only one person I could trust and believe in, my younger brother."

Mr Britton said "every other person on the planet was a suspect".

"That's how we felt ... we couldn't trust anybody."

Police are still hoping a crucial witness might emerge, saying relationships and loyalties change over time.

"If people had information 22 years ago - and for a variety of reasons didn't feel comfortable or safe coming forward and talking to us about it then - now is the opportunity to do so," homicide detective Darren Mills said.

He said the protective dog and other factors suggested the killer or killers had intimate knowledge of Dorothy's lifestyle, movements, and her property.

"(There was) definitely some familiarity with the family dog, which was described quite widely at the time as very, very protective of the property, but in particular very protective of Dorothy."

Dorothy's body was found on March 7, 1996 after relatives could not reach her and went to check on her. She was last seen alive about midday the previous day.

When they arrived, the front door was open, and the dog was standing on the verandah. There were no signs of a struggle and Dorothy's handbag, containing cash, was not touched. A shotgun missing from the property has never been found.

A $250,000 reward remains on offer.

Australian Associated Press