A contest mention is a step that is used to resolve matters that are going to be a contested hearing / plea of not guilty.
The informant is expected to attend as are you and your lawyer.
Your lawyer will have read the police brief, got instructions from you about what you want to happen and be able to advise the Magistrate what the issues in the case are.
The prosecutor and your lawyer will discuss whether the matter can be resolved before you get into Court. Charges are often withdrawn at contest mention as the Police may realise that they can not prove them or that what they are alleging did not happen.
The Magistrates will often give a sentence indication in a contest mention. This is where they will indicate to you what penalty you will get if you plead guilty at that stage. The aim is to save the Court the further time of other hearings of your matter.
You are in no way obliged to plead guilty and the Magistrate who hears the contest mention is not meant to hear the contested hearing. Often the Magistrate will give a common sense evaluation of the strengths and weaknesses of the evidence against you.
At this stage you lawyer will indicate how many witnesses will be called (if any) and whether you are disputing specific pieces of police evidence, such as a record of the interview the Police had with you.