Found the law applicable to middle fingers..
Intentional harassment, alarm or distress
Miscellaneous Offences Act 13A.
—(1) Any person who in a public place or in a private place, with intent to cause harassment, alarm or distress to another person —
(a) uses threatening, abusive or insulting words or behaviour; or
(b) displays any writing, sign or other visible representation which is threatening, abusive or insulting,
thereby causing that person or any other person harassment, alarm or distress, shall be guilty of an offence and shall be liable on conviction to a fine not exceeding $5,000.