Sydney man denied bail over homemade bomb

A Sydney man accused of possessing a potentially "catastrophic" homemade bomb wrapped with 50 ball bearings has been refused bail in court.

Harley Roy Turnbull, 22, was arrested on Wednesday after his mother allegedly found the device in his bedroom at their home in the western Sydney suburb of Werrington County on Tuesday night.

Bomb squad specialists later detonated it at a nearby football field.

Turnbull was charged with possessing a prohibited weapon and an explosive for unlawful purpose.

On Thursday, Turnbull appeared via audio visual link in Penrith Local Court where he was refused bail after his lawyer, Brad Mallinson, entered pleas of not guilty on his behalf.

In refusing bail, Magistrate Dorelle Pinch took into account previous failures by Turnbull to appear in court, his lengthy criminal record and the serious nature of the offence.

"There are no bail conditions that can mitigate these unacceptable risks," Ms Pinch told the court.

After the decision, Turnbull, sporting a shaved head and facial tattoos, swore at the magistrate before his audio visual link was turned off.

Earlier, the court heard police held "grave concerns" for the community if Turnbull were released.

Prosecutor Ben Seabury said the device had the potential to "kill or maim", adding that "50 ball bearings would have caused major damage".

Earlier this week police said the bomb had the potential to be catastrophic.

The court was told Turnbull obtained the "improvised explosive device", allegedly wrapped with ball bearings and contained in a bag, from a silver car outside his house. It's alleged he then took the bag into his bedroom and later left in the car.

Turnbull's mother went into the bedroom, opened the bag, and called police.

The court heard that when shown a picture of the homemade bomb in an interview with police after his arrest, Turnbull said the device "looked cool" and "could be a bunger".

Mr Seabury conceded there was no evidence to prove Turnbull saw the device "before he was shown the picture ... during the interview".

In arguing for bail, Mr Mallinson conceded his client had a "dreadful" criminal record, but said he didn't know "what he was in possession of".

"If he knew what it was, he would have been far more careful," Mr Mallinson said.

The matter returns to Parramatta Local Court on February 27.