A NSW police officer is in a serious condition in hospital after being stabbed and slashed with a makeshift spear while on a routine job in Sydney’s south-west.

Constable Jacob Vella, 24, was allegedly attacked by the son of a man he was seeking to arrest on a warrant while attending a property with his partner in Sadleir, near Liverpool, just before 7am on Tuesday.

The plain-clothed officers were confronted inside the home by a 27-year-old, who then allegedly stabbed Const Vella in the hand and slashed him on the neck.

The injured constable was rushed to Liverpool Hospital for emergency surgery.

He remains in a serious but stable condition.

Assistant Commissioner Peter Thurtell said the weapon appeared to be homemade, comprised of a steel shaft on the end of a wooden stick.

“It was a crude weapon,” he said in Sydney on Tuesday.

“We’re hoping for the good and speedy recovery of the young police officer.”

The alleged attacker was arrested at the scene and is likely to be charged with attempted murder.

His 58-year-old father, who was named on the warrant for failing to appear in court over breaches of a community corrections order, is also in custody and expected to be charged.

The pair are known to police but not for violence-related matters, Mr Thurtell said.

A third man, who was inside the home at the time, is speaking with investigators.

Constable Vella has been in the force only since April 2017.

Mr Thurtell said this was the latest in a string of recent incidents where police officers have been threatened or wounded.

“It’s a dangerous profession – but it’s only dangerous because there’s people out there who don’t respect the police uniform and who think it’s alright to attack police officers,” he said.

“It’s not all right at any stage to attack anybody.”

The Police Association of NSW president Tony King described the attack as “gut wrenching”, especially given the type of weapon involved.

“It’s concerning to see incidents like this occur when police are doing routine jobs like arresting people for warrants,” he told reporters in Sydney.

“There certainly seems to a rise in disrespect and assaults on police.”

Mr King said, on average, there were 50 assaults against police every week in NSW.

-AAP