A 22-year-old driver who allegedly struck two police officers at a random breath test site in Sydney's south-west, leaving them with serious injuries, has been granted bail after a court heard he had been suffering from post-traumatic stress disorder.

Jakob Thornton is facing two charges of dangerous driving occasioning grievous bodily harm after hitting senior constables Jonathan Wright and Matthew Foley at Leumeah in February while he was using his mobile phone.

The injuries to Senior Constable Wright's right leg were so serious he had to have his foot and the lower part of his leg amputated.

In the NSW Supreme Court on Thursday, Justice Peter Hamill said the charges Mr Thornton was facing were "very serious and usually, though not always, result in a full-time custodial sentence".

Despite this, Justice Hamill granted the 22-year-old bail with strict conditions including a $5,000 surety put up by his mother who was in court to support him.

Mrs Thornton cried as her son was granted bail and did not answer questions outside court.

Mobile phone would 'constitute dangerous driving'

Justice Hamill told the court it was alleged that Jakob Thornton was using his mobile phone while driving, which could "constitute dangerous driving".

"Unfortunately, in the course of his driving he collided with two police officers who were stationary in lane one of a major road that Mr Thornton was driving down," Justice Hamill said.

"The Crown case appears to be a reasonably strong one … the injuries suffered by the police officers were serious and on their face would constitute grievous bodily harm."

Mr Thornton's lawyer said his client was suffering from symptoms of post-traumatic stress disorder and planned to see a psychologist when he was released on bail.

Justice Hamill ordered the young man to attend counselling and provide proof of his appointments when reporting for bail.

The court was told Mr Thornton has a history of driving offences and a previous assault conviction that was overturned.

"The reality is whilst the young man has a fairly bad traffic record, it is also the case that when his licence has been suspended he has complied with that suspension and it seems he has no record of non-compliance in that regard," Justice Hamill said.

Mr Thornton has not yet entered a plea, and is due to appear in Campbelltown Local Court in May when police are expected to indicate when their brief of evidence will be ready.

System has let community down: police union

NSW Police Association president Scott Weber said police officers were "devastated" over the decision to grant bail.

"They [the police officers involved] have permanent injuries for the rest of their lives," Mr Weber said.

"The system has let down not only the police officers involved but the community as well."

A NSW Police spokesman said "both officers conditions are improving and the NSW Police Force looks forward to them returning to work as soon as possible".