Officer's arm broken during Sydney arrest

A teenager has been charged after allegedly breaking a female police officer's arm after a home invasion in Sydney.

Four men, aged 19 and 20 and allegedly armed with iron bars, assaulted another man outside his Petersham unit before forcing entry and ransacking it early on Saturday morning, police say.

The group fled and one of the 19-year-olds was arrested nearby, but not before he allegedly threw an officer to the ground and caused her to break her arm.

The others were arrested elsewhere, with all four refused bail to appear in Parramatta Bail Court on Saturday charged with aggravated break and enter, malicious damage and assault causing actual bodily harm.

The man who allegedly attacked the officer was also charged with resisting arrest and assault causing grievous bodily harm.

The officer will undergo surgery.

Four police officers have been injured on the job in NSW in the past 24 hours.