A dialogue coach is the person who helps actors and actresses pronounce and enunciate their lines with certain emotions to give it the right touch. Even more importantly, a good dialogue coach helps actors master difficult accents and dialects for more specific roles.
Being a dialogue coach can sometimes be a challenge because it takes a lot of knowledge and communication skills to help another person physically perform something with their own mouth, as opposed to their hands or bodies. For you it may be easy to do an Australian accent, but try explaining to someone how to do it!
Dialogue coaches usually have a degree in English, speech communication, or theater. Many of them are successful actors in addition to their work. As a general rule, dialogue coaches in films are usually hired for their ability to quickly figure out what's wrong with a particular actor's accent or character.
On occasion, this can leads to all sorts of confrontation. Somewhat infamously, Tim Roth grew so exasperated with his dialogue coach on the set of Reservoir Dogs that he cajoled Quentin Tarantino into letting his coach play one of his victims on-screen, just so he could kill her and vent a little!