The 12-person jury returned the verdict for a charge of murder on Friday afternoon after three days of deliberation following a two-week trial in the Supreme Court in Wollongong.

The jury heard Matthew Henry Spinks, 35, fired two gunshots from his slow-moving car through Mr Costello's parked car in the early hours of February 14, 2018 outside Carr Parade, Unanderra.

Read more: The high-flying twin brothers charged over $14.7 mil Avondale 'development scam'

The jury retired to deliberate their verdict on Tuesday afternoon.

On Friday, about midday, the jury sent Justice Peter Hamill a note saying they could not reach a unanimous verdict however about 10 minutes later, jurors returned to the courtroom to say they had reached a decision.

Justice Hamill thanked the jury for their civil service and formally discharged the seven women and five men.

The Crown alleged Spinks deliberately shot Costello in "revenge" after he went to Spinks' Koonawarra home on February 5, 2018, threatened him, armed with a knife.

The jury heard Spinks and Costello, who was schizophrenic, were friends before they had a falling out.

Crown prosecutor David Scully told the jury Costello and his partner Melanie Hazell went to his friend's Unanderra home so his friend, Daniel Blow, could fix his car.

At about 2.15am, Costello was standing on the grass verge between his car and the fence of the house when a black Mercedes SUV allegedly slowly pulled up along Costello's car, after circling the area several times.

The Crown alleges Spinks pulled out a 12-gauge shotgun and fired two shots, with the pellets hitting Costello in the face, neck, chest and shoulder while the other shot landed in the car.

Ms Hazell told the court in her evidence that after hearing the loud bangs, she got out of the car and Mr Blow pulled her down and into the house, where he started locking doors and told her to be quiet.

She said she called triple-0, then threw the phone to Mr Blow's partner and ran back outside to Mr Costello.

Ms Hazell said she wanted him to sit up because he had blood in his mouth but she couldn't hold him up, so she dragged him towards the fence to prop him up.

Costello died from his injuries and a crime scene was established.

Mr Scully said the shot went through one window, out the other side of the almost fully open rear passenger side window and hit Costello, who was leaning over.

When on the stand, Spinks told the court he drove to Carr Parade and turned the headlights off before he turned into Richards Street, where Mr Costello's car was parked.

He said he had a clear view of Mr Costello standing on the other side of the car, towards the front.

Spinks said he shot towards the car's back window, saw Mr Costello duck, then fired again.

He told the court he aimed both shots downwards, through the back window into the vehicle's back seat.

Spinks then drove away slowly before it accelerated harshly.

Police found the car dumped and burned at Noonga Reserve in Cordeaux Heights a day later, with Spinks fingerprints found inside. He was later arrested.

In court, Spinks returned to custody and he will be sentenced at a later date.

We depend on subscription revenue to support our journalism. If you are able, please subscribe here. If you are already a subscriber, thank you for your support.