Police say they won’t apologise after dropping terror charges against Mohamed Kamer Nizamdeen, a university student who was accused of an assassination plot targeting Australian politicians.

Nizamdeen, a 25-year-old PhD student, was arrested in August and accused of plotting Islamic State-inspired lone-wolf attacks targeting the Opera House, other notable landmarks, and the former prime minister Malcolm Turnbull.

The prosecution relied almost exclusively on a notebook police said contained details of the plot.

But the evidence against Nizamdeen was fundamentally flawed, according to the accused’s lawyers. Handwriting experts were unable to link the writing in the notebook to Nizamdeen.

Nizamdeen’s lawyers had earlier described the case as “extremely weak, almost non-existent”, while successfully arguing for bail last month.

The matter appeared again in the Central local court on Friday. Prosecutors dropped the charges and withdrew the case.

Australian federal police assistant commissioner Ian McCartney later told reporters “at this stage, based on the evidence we’ve got, it’s likely he did not write those comments in the notebook”.

Speaking outside court, Nizamdeen’s lawyer, Moustafa Kheir, flagged potential civil action over his client’s treatment. He said he would also apply for his legal costs.

“What authorities have done to this young man is absolutely unforgivable,” Kheir told reporters outside the court. “We will be seeking justice for him in the NSW supreme court.

“It’s a terrible experience, as a young man who has done everything right in life, he has gone through supermax jail in unforgivable circumstances.”

But police declined to apologise to Nizamdeen, who’s planning to sue for compensation after being detained in Goulburn’s supermax jail.

NSW police assistant commissioner Mick Willing denied police had ruined the young man’s life stating “those who were involved in the production and manufacture of (the notebook) are the ones who’ve had an impact on Mr Nizamdeen”.

He said the investigation was ongoing because “there were very serious threats against individuals contained within that document”.

The senior police argued investigators had acted in good faith and notified prosecutors as soon as they realised there could be an issue with the evidence.

Asked if Nizamdeen had been framed, McCartney replied: “There are a number of lines of inquiry in relation to this investigation.”

Both the AFP and NSW police refused to apologise.

Nizamdeen’s costs application was set down for hearing on 23 November, his lawyer told reporters.

Nizamdeen spent about last four weeks in jail before being bailed last month. His family and supporters maintained his innocence, and the case prompted protests in his hometown in Sri Lanka.

Nizamdeen is in Australia on a student visa while he completes his studies.

He had no criminal history, and he was promoted by the University of New South Wales in its advertising. Nizamdeen also worked as a contractor for the university.

A court earlier heard that no extremist material was found on his computer or mobile phone.

After he was bailed, Nizamdeen’s lawyer Tweeted that the police case was “hopeless”.

“Bail granted to my client Mohamad Nizamdeen today,” he wrote.

“Police case is hopeless, as notes in question are not his! We will continue to seek justice until my client is fully exonerated.”

Nizamdeen’s arrest came after a tip-off from a worker at the university, police said in August.