I am here as a visitor but I have seen adverts for work visas and an agent has offered to make an application for me. Is this OK?

Usually no. There are a number of agents who offer to get work permits for visitors. In fact they make an application for a Protection visa which is asking for refugee status. There are very few genuine refugees and almost all applications for Protection visas are rejected within 28 days. Then the Agent applies to the Refugee Tribunal and it may take 6 to 9 months for a hearing. You will be rejected and have to pay an application fee of A$1,400. You will also find it very difficult to get a temporary visa, that is another visitor, a student or temporary business visa, to come back to Australia during the next 5 years.