The question that has stumped Marion Barter's family and friends for decades is now before NSW State Coroner Teresa O'Sullivan, who on Monday opened an inquest into the former NSW and Queensland teacher's disappearance.

Ms Barter has not been seen since leaving Australia for an indefinite trip to England on June 22, 1997.

But her relatives and police now know she'd already changed her name by deed poll to Florabella Natalia Marion Remakel and would soon slip back into Australia.

The coroner was told Sally Leydon reported her mother missing in Byron Bay October 1997, noting her suspicion Ms Barter was back in the country.

But police did little for 10 years, having marked Ms Leydon's report "occurrence only" and "no further action required" in the first 36 hours.

Detective Senior Constable Gary Sheehan picked up the case in 2009 and said the police narrative was "very, very scant in detail" and the file lacked a picture of Ms Barter or a detailed statement from Ms Leydon.

The "occurrence only" marking was "inappropriate" in light of the available information, he said.

Before going to police in 1997, Ms Leydon was told by a Commonwealth Bank employee a series of $5000 withdrawals had been made from her mother's account.

But the transactions records no longer exist, the inquest was told.

Limited documentary evidence includes proof her Medicare card was used at a Grafton optometrist on August 13, six days after her RACQ membership and policy was cancelled over the phone or at a branch.

Det Sen Const Sheehan defended suspending the case in 2011, causing Ms Barter to be marked "located" despite no sighting or proof of life.

He decided Ms Barter went "to great lengths" to estrange herself from her family and start a new life, including calling and writing to family from England in June and July 1997, he said.

"She was keeping up a front on one hand but also covertly arranging herself a new life," he told the inquest.

"It's not the job of the police to find out why, it's the job to find out what happened."

He denied it was inappropriate to mark an unsighted person "located", saying there were no fears for the teacher's safety.

Ms Barter, who would now be aged 75, married three times, including first to Australian soccer legend Johnny Warren.

From the 1960s, she taught at various locations including the Blue Mountains and the NSW South Coast before taking a job in 1994 at prestigious The Southport School on the Gold Coast.

But Ms Barter quit midyear in 1997, sold her house at a loss, packed her belongings in an unknown storage unit and - without her family's knowledge - changed her name to the "highly unusual" Florabella Remakel, counsel assisting Adam Casselden SC said.

"It is likely that the reasons behind her change of name are intrinsically connected with the mysterious circumstances of her disappearance and the question of whether she is alive or deceased," he said in his opening address.

Just before leaving Australia, Ms Barter was spotted late one night in May 1997 at a petrol station. The man seen in the passenger seat of her car has never been identified.

Mr Casselden said the investigation was "very much ... ongoing".

"If any member of the public has any information ... I strongly encourage you to come forward," he said.

The inquest is due to run two weeks, including hearings next week in Ballina and Byron Bay.

Australian Associated Press