The detective who led the police investigation into convicted murderer Keli Lane has broken her silence, revealing she was "shocked" when Lane was found guilty.

Speaking publicly for the first time since Lane was convicted of murder in 2010, former detective senior constable Sharon Rhodes has told the ABC's documentary series Exposed that the trial had been plagued with problems and the jury did not hear from people who she believes are crucial witnesses.

Ms Rhodes has also revealed the prosecution case was being prepared on the run, and the trial was marred with the constant late tendering of evidence.

Key points: Lead detective in Keli Lane investigation says she was stunned Lane was found guilty

Lead detective in Keli Lane investigation says she was stunned Lane was found guilty Says the police investigation fraught with dead ends

Says the police investigation fraught with dead ends Lane is in jail for the murder of her newborn baby Tegan

"I was stunned. I'm surprised they [the jury] were able to get to that point," Ms Rhodes said.

"I honestly thought that she would be found not guilty because there had been so many problems."

Ms Rhodes, who has since left the NSW Police, has also told the ABC that the murder investigation was incomplete at the start of the trial and police struggled with a lack of leads and multiple dead ends.

A detective with the Unsolved Homicides unit, Ms Rhodes was assigned to the Keli Lane case in November 2006 and investigated her exhaustively for four years.

While she believes Lane murdered her baby Tegan, she questions whether the trial that led to Lane's conviction was fair and just.

"I've got no doubt in my mind that she committed the crime, that she killed Tegan, murdered her. But she's an accused person. She deserved due process," Ms Rhodes said.

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Police used every trick in the book

A still from video of detective Sharon Rhodes at the site of the excavation ( ABC: Supplied )

Ms Rhodes said that despite public statements to the contrary, police produced a search warrant and excavated a residence in Gladesville based on a hunch, rather than any new lead.

"I typed up the warrant with the belief that if Tegan was anywhere, that Keli had taken her to Venus Street and disposed of her remains there," she said.

But after an extensive search, Tegan's remains were not found.

In another tactic deployed by police, an undercover operative attempted to befriend Lane in order to extract information about baby Tegan's fate.

Exposed can also reveal police covertly collected DNA samples from Lane and members of her family, but police bungled the operation and mistakenly mixed the DNA samples.

Lane was also the subject of extensive surveillance including hundreds of phone intercepts and listening device recordings.

"Didn't bear us any fruit unfortunately," Ms Rhodes said.

"But we tried. We really tried. Every resource that was available. We didn't have anything."

Internal police documents reveal lack of motive

Internal documents obtained by Exposed reveal that after a police investigation spanning eight years and a coronial inquest, by November 2008 the Unsolved Homicide Squad was internally questioning whether they had enough evidence to prosecute Lane.

"To be completely honest, we hadn't reached the standard of beyond a reasonable doubt, so we weren't in a position to charge Keli Lane with murder," Ms Rhodes said.

The Unsolved Homicide Squad referred the matter to the NSW DPP for a legal advising on the police case, which was based on circumstantial evidence.

The legal advising was overseen by the Director of the NSW DPP, Nicholas Cowdery QC.

Nicholas Cowdery QC was the Director of the NSW DPP. ( ABC News )

There was enough evidence to justify charging: Cowdery

Mr Cowdery has told Exposed that at the time the Lane matter was referred to the NSW DPP, his office was under stress financially and politically.

"This period, 2009/10, was one of those periods where we were dealing with a lot of those kinds of pressures," he said.

Mr Cowdery said that against the backdrop of financial and political pressure, it became necessary for the NSW DPP to prioritise high-profile cases including Lane's matter.

"That's not ideal. It's not in the general public interest to have to do that, but sometimes you have to," he said.

Mr Cowdery said he carefully evaluated the circumstantial evidence before making the decision to charge Lane with murder.

"What I was looking for to support a murder charge was circumstantial evidence," he said.

"Evidence of events surrounding the disappearance of the child that pointed to culpable involvement of Keli Lane.

"There were the lies, of course, that Keli Lane told and which were able to be proved, they were significant.

"There were improbabilities in some of the accounts that she had given."

He said the circumstantial evidence was strong and there was enough to charge Lane with the murder of Tegan, along with three counts of perjury.

"At some point, there was enough evidence to justify charging," he said.

"People sometimes quiz you about the nature of a circumstantial case.

"It sounds as though it's some sort of a guess or some sort of speculation about what might have happened, but that couldn't be further from the truth.

"A circumstantial case can be stronger than a case that is supported by direct evidence."

It's not the due process: Rhodes

Behind the scenes, the 2010 murder trial presented its own set of problems.

There was a "mammoth" amount of witness statements and complex searching still being done by police throughout the trial at the request of the prosecution and queries by the defence, known as requisitions.

Sharon Rhodes was the lead detective in the Keli Lane case. For the first time she is speaking about the case and investigation. ( ABC )

This led to a logjam of evidence being tendered late, Ms Rhodes said.

"As the trial went on those requisitions were coming in thick and fast," she said.

"There's always one or two things that need to get served as an additional statement or something will come up and that's not unusual, that's fine.

"But these were hundreds and hundreds and hundreds and hundreds of requisitions. Hundreds."

Due to the scale of late tendering of evidence, she believes the defence didn't have enough time to digest the information.

"So much material was being served on the defence during the trial, and the defence were getting very upset about it. Understandably so," Ms Rhodes said.

"They couldn't go through the material and prepare for the trial for the next day. It's not the due process. It's not the system that we work in."

Keli Lane's defence team declined an invitation to be interviewed by Exposed.

A case 'prepared on the run'

The presiding judge, Anthony Whealy QC, said Lane's trial was highly stressful and unusual because of the amount of evidence submitted during the trial.

Judge Anthony Whealy presided over the Keli Lane trial in 2010. ( ABC: Supplied )

"It was unusual in that, as the trial went on, more and more evidence was produced," Mr Whealy said.

"That was very unusual."

Mr Whealy also confirmed that the police investigation was incomplete by the time the matter progressed to trial.

"It should have been completed before this trial began," he said.

"This was a case that was being prepared on the run. There's no doubt about that."

A culmination of a high-pressure case combined with an extraordinary volume of work took its toll on the lead detective.

This was the one that broke me

Sharon Rhodes (left) says being given the case felt like a reward, but it became unbearable.

Ms Rhodes was a decorated detective with a string of commendations to her name and multiple criminal trials under her belt when she took on the Keli Lane investigation.

She said that being given carriage of the investigation "felt like a reward".

But the high-profile nature of the case and the pressures of the investigative work required during the trial became unbearable.

"I've worked at Homicide, I've worked at Child Protection, I've worked at Unsolved Homicide, and I've seen a lot of things, experienced a lot of things," she said.

"There was a lot of pressure with this. More pressure than I'd ever felt with any other matter."

Ms Rhodes' Police Duty Books, which are a contemporaneous account of an officer's daily work, detail her stress over the course of the trial.

"It was hell. It was torture. I hated it. By the end I hated it. I hated going to work, but I had to go, so I went. I white knuckled it."

After her final day in the witness box in November 2010, Ms Rhodes left court and didn't return to work for NSW Police.

"I never set foot in that building again. Never set foot in State Crime or at the court again," she said.

Ms Rhodes has built a new life for herself with a new career, but still struggles with the fact Tegan has never been found.

"I think Tegan has been forgotten as such, a lot, and it became more about the process and the criminal justice system, and Tegan was just the name on the charge sheet," Ms Rhodes told Exposed.

"She still haunts me. It's still unsolved. Nobody has ever found Tegan."

The three-part documentary series EXPOSED will air on ABC, the third episode will air at 8:30pm AEDT on Tuesday, October 9 and is available on iView.