A man who cable-tied the hands of three children in Western Australia's far north has been found guilty of two counts of aggravated common assault.

Matej Radelic, 46, faced a single-day trial in the Broome Magistrates Court last month, after pleading not guilty to three counts of aggravated common assault in April. 

He used the ties to restrain the children — aged six, six and seven — after discovering them swimming in his pool at a property in Cable Beach on March 5.

Video of the incident, showing children with their hands bound together, made international headlines.

On Friday, Magistrate Deen Potter found Radelic guilty of two of the three charges, committed against two of the children. 

He was acquitted of the third charge, as one of the children was restrained for a shorter amount of time and able to leave the scene once he broke free.

Force became 'unreasonable'

A key point in the magistrate's decision was whether Radelic's use of force was "reasonably necessary" in the circumstances.

He deemed it was not given the length of time — 37 minutes —  two of the children were restrained for before police arrived.

Describing the 46 year old as a "towering figure", Magistrate Potter said the children's distress was clear.

He said the images circulating of the children bound together had been "seen in the historical context", which had a "destabilising effect in the community."

Magistrate Potter said the incident had "a marked impact" on the children and their family, because of that context.

"While he was on the telephone he described the children as being scared and crying," he said.

"Their distress should have been an indication.

"Therefore at some point, [it must have] become blatantly clear that the continued application of force must have become unreasonable.

"There's an inescapable fact that the children were very and clearly distressed by their captivity."

Magistrate Potter did acknowledge Radelic had experienced previous break ins on his property, and that he was responding to what he perceived as trespass and criminal damage leading him to conduct a citizen's arrest.

But he said there was no suggestion the three children were connected to the previous incidents.

"This wasn't born out of any sense of vigilantism," he said.

The magistrate handed Radelic a $2,000 fine, which was suspended for a 12-month period, and a spent conviction.

Defence argues a 'lawful' citizen's arrest 

During last month's trial, both prosecution and defence agreed on a statement of material facts, acknowledging Radelic had bound the children's hands, and the children, regardless of their age, had carried out "trespass and criminal damage" to the 46-year-old's property.

The trial largely focused on whether Radelic's decision to restrain the children with cable ties was "reasonable and necessary". 

Radelic's lawyer Seamus Rafferty told the court that his client had been a "victim of crime" after his home was broken into on multiple occasions prior to the incident, but did not suggest it was the same children. 

He told the court that Mr Radelic had carried out a "lawful" citizen's arrest when deciding to restrain the three children, and it was not his client's fault that it took police 37 minutes to arrive at the property after he had phoned them. 

During sentencing submissions, Mr Rafferty told the court the incident had a significant impact on his client's family including a loss of business.

But police prosecutor Mícheál Gregg argued that while Radelic may have carried out a lawful citizen's arrest, it was unreasonable for him to have cable-tied the young children who had been compliant with his instructions. 

Sergeant Gregg described Radelic's actions as having a "dehumanising effect" on the children and said he had other options available to him at the time. 

"[It] catapults it into the unreasonable," he said.