A man and a woman have been arrested in the Illawarra south of Sydney over an alleged car bombing plot last month.

Police will allege the pair was involved in a minor “benign” altercation with a couple previously unknown to them in a Warrawong car park, south of Wollongong, in late February.

The Fixated Persons Investigations Unit began investigating after the alleged victims received a barrage of online abuse.

Their car, which was unoccupied at the time, was parked in the driveway of their Koonawarra home when it was allegedly bombed.

The bombing in early March partially destroyed the car, said Detective Superintendent Michael McLean from the Fixated Persons Investigations Unit.

“This was an act of grievance-fuelled violence … The victim of this matter was an unintended victim, was not known to the offenders,” Detective McLean said.

“And what we will allege followed was a barrage of online threats, harassment and intimidation, which ultimately led to the placement of this … improvised explosive device.”

Early on Tuesday detectives joined by the Riot Squad arrested a 33-year-old man from Lake Heights.

That was followed by the arrest of his 31-year-old female partner at Lake Heights a short time later.

Police are still questioning the couple, who are expected to be charged with a number of offences including using an explosive with an intent to harm, using a carriage to menace, harass and offend and stalking and intimidation.

The Fixated Persons Investigations Unit was set up by NSW Police two years ago to focus on the so-called “lone actor” and “fixated person” threats and grievance-fuelled violence.

–ABC