The alleged attack occurred after officers broke up a fight between the 23-year-old and a woman known to him outside a home in Lansvale, near Liverpool.
Police were called to the Huntingdale Avenue property at 1.10am over a noise complaint.
When they arrived, they found the pair, both aged 23, arguing outside the home.
The row became physical and police intervened, separating them.
While officers were speaking to the woman, the accused approached a male sergeant and confronted him.
Police allege he then punched the sergeant in the jaw, knocking him backwards and causing him to hit his head on the ground.
The sergeant was unconscious for a short period and was treated by colleagues at the scene before paramedics arrived.
He had a cut on the back of his head and facial swelling and was taken to Liverpool Hospital in a stable condition.
As the alleged offender was arrested, police say a probationary constable was pushed.
The Chester Hill man has been refused bail and is scheduled to appear at Liverpool Local Court tomorrow.