Mr Zumbo was office manager for Mr Kelly, a former Liberal MP who recently moved to the cross-bench.
Police will allege he sexually touched and indecently assaulted the girl and three women aged 23, 26 and 27 on multiple occasions at Mr Kelly's Sutherland electorate office between 2014 and 2020.
He was arrested at Sutherland Police Station shortly before 6pm on Friday.
He faces nine counts of common assault, two counts of aggravated sexual touching and seven counts of aggravated indecent assault of a victim under his authority.
Zumbo 'categorically denies all accusations'
Mr Kelly on Saturday said every citizen was entitled to the presumption of innocence and protection by the rule of law.
"I have spoken with Mr Zumbo and he advises that he categorically denies all accusations and he will vigorously defend the matter," Mr Kelly said.
He said Mr Zumbo was looking forward to the opportunity to clear his name in the courts after "12 months of innuendo and rumour".
Mr Kelly added that the complainants did not bring their accusations to him and he only learned of them in the media.
He has not been interviewed by police, he said.
Mr Zumbo will appear before Sutherland Local Court on 24 June.
His bail conditions prohibit him entering the grounds of educational institutions like schools and universities, and contacting prosecution witnesses.
The investigation is continuing, police say.