A report from a former colleague of Dr Coop, who said for the first time last year she saw him in Australia in 1997, prompted police to make new inquiries across the Tasman, Senior Sergeant Bruce Ross, of Dunedin, said.
So far, those inquiries had come up with nothing, but are ongoing.
Dr Coop, then 28, went missing on Queen's Birthday weekend 1989 after flying to Dunedin from his home in Auckland to attend a month-long postgraduate course for aspiring eye specialists.
A massive search failed to find him, and various sightings over the years have led nowhere.
In September last year, on a television show on Dr Coop in which former colleagues of the man were interviewed, Australian doctor Elizabeth Hagen, who studied with Dr Coop, revealed one of the most credible possible sightings yet.
She did not go into specific details on the show, but in a letter detailing the sighting to police, which police received about the time the show aired, she reported she believed she saw Dr Coop at a campsite in a rural area of north New South Wales in 1997.
Dr Hagen said she and her husband were both shocked when they recognised Dr Coop as he pulled over in his vehicle, a distinctive yellow Lada, to let them pass on a narrow driving track.
A woman and two children were also in the car with him. Later that day, they drove past the man, woman and children beside a campfire at the camping ground near where they were staying.
In a quandary about what to do, Dr Hagen said she decided to sleep on it, but after a restless night, got up at 5.30am and walked to the campsite to confront the man, but the campsite was deserted.
She told police she kept silent for so long because she had respected her old friend's right to choose his life, even though she was conscious his family and friends were left with no resolution and conclusion.
She was also concerned the information that Dr Coop was possibly living a new life in Australia could be hurtful to his relatives.
The family has since been made aware of the details of Dr Hagen's 15-year-old sighting.
Police believed the sighting was credible enough to follow up, and the alleged night-time vacation of the campsite seemed suspicious, Snr Sgt Ross said.
He was still attempting to narrow down dates with Dr Hagen and had contacted the then-owner of the camping ground, seeking records of vehicles that were registered there during that time.
However, because the business had gone bankrupt, it was not certain where the records were.
He was now trying to track down lawyers who might have them.
Dr Coop's sister, Diana Coop, of Wellington, encouraged anyone with information to assist police as much as they could.
It was great Dr Hagen had finally spoken up, although it was why she had not said anything earlier that had been confusing at the time, Ms Coop said.
"It was very odd someone external to the family might have had that information and not passed it on to [the family]."
She was pleased the police were actively following up what seemed like a credible sighting.
The family's view was that everything was worth following up, "because we just have nothing".
Her brother's disappearance was something the family lived with constantly.
In recent years, a lot of the family's energy on the matter went into a charitable foundation providing support information for the families of missing people, which they started in 2008.
"For every person who goes missing, there are at least 20 people who are dramatically affected. There are a lot of complications for people, and this is something we are doing to help."
