[9] The principal ground of appeal before Wylie DCJ, and again here, is that the written warning on the "Identifying Particulars Notice" is insufficient to satisfy the requirements of s 57(3) that a police officer warn the person. The first question however is whether absence of proof of a warning under s 57(3) is fatal to such a prosecution. Mr Winn, counsel for the respondent police constable, submitted that a failure to warn would be a mere failure of duty on the part of a police officer to which no particular consequences were likely to be attached, except perhaps possible discipline. However in my view the warning is an essential part of a valid requirement under s 57(1)(b)(ii), and should be regarded as an ingredient of the offence. Furthermore, the offence under s 120(1) is obstruction of a police officer "in the performance of the officer's duties". If a contravention occurs following a police officer breaching his express duty to warn the other person under s 57(3), it would be difficult to say that the obstruction was one in the performance of the officer's duties. The relevant duty would be to make a proper requirement under s 57(1)(b)(ii), and in my view that involves the police officer's own compliance with s 57(3).