Ryan Sharp and his partner of 18 months, Deanne Struber, were arrested in April last year after the pair carried out a six-week campaign of harassment against the Koonawarra couple that began on the afternoon of February 22 with the beep of a horn.

A set of agreed facts tendered to Wollongong District Court on Tuesday said Struber was in the McDonalds drive-thru at Warrawong when she began reversing her vehicle, prompting the victims, who were in the car behind, to beep their horn.

Struber verbally abused the victims before driving off, however both vehicles stopped a short distance down the road and an altercation occurred.

Struber reported the matter to police and was able to obtain the male victim's name and suburb. She phoned Sharp who told her to look up the couple on social media. Struber discovered their joint Facebook page and sent Sharp a screenshot.

Sharp then created a fake Facebook account with an image of a young blonde woman and contacted the victims, inquiring about furniture they had listed for sale on Marketplace.

The victims provided Sharp with their address.

CCTV recorded Sharp attend the Koonawarra home early the next morning and smash up the couple's grey Ford Ranger with a baseball bat, causing about $10,000 damage.

He then sent the pair a message threatening to set their car on fire and rape the female victim, saying "both of you have made a big mistake".

Two weeks later, Sharp again attended the couple's house, this time planting a bomb made up of sparklers, batteries and a circuit board under another vehicle at the premises. It detonated less than 90 minutes later.

When arrested, Sharp initially denied any involvement in the offences but later admitted to creating the explosive device.

He also admitted sending the Facebook posts, saying he was frustrated police hadn't acted on the road rage incident and had decided to take matters into his own hands.

Sharp pleaded guilty to using a prohibited weapon, manufacturing a military style weapon and intimidation charges.

In court on Tuesday, defence barrister Scott Fraser said his client had not intended to hurt the victims with the bomb, but only to scare them.

"Ultimately the risk that was in fact created appears to be relatively low, and the device - as devices of its type go - is above the bunger situation but significantly well below other devices," Mr Fraser said.

"[But] it has been accepted this is an offence that is serious, and serious enough that Mr Sharp is expecting a full time custodial sentence."

A psychological report revealed Sharp had shown little remorse for his actions and had been very "self-focussed" from a young age.

Meanwhile, in a statement tendered in court, the victims said they had had to move house a number of times during the police investigation and had been left in fear of their safety.

"It put us on edge... we did not know him but he seemed to know everything about us," the couple wrote.

"Every time saw a strange car in the street we became anxious.....eventually we had to sell our home.

"We've had to start our lives from scratch."

Judge Andrew Haesler said a clear message needed to be sent that if citizens took the law into their own hands, they would face significant punishment.

"This was a concerted attempt to intimidate the [victims]," he said.

"He persisted using a variety of means...the offences were planned and the victims tracked on a number of occasions. There was a high degree of premeditation."

Judge Haesler accepted that Sharp's device had been a crude one, designed to scare but not cause physical harm, but said nothing could justify or explain the extraordinary actions Sharp had taken in planting the bomb under the car after his campaign of harassment against the family.

"The device was not just made, it was used," he said.

"This was an escalating series of acts designed to cause psychological harm to the victims."

Judge Haesler set a non-parole period of three years, meaning with time served, Sharp will be eligible to apply for parole in June 2022.