What is the best treatment for a mouth ulcer? (available in australia)

Depends: Mouth "ulcers" is a very nonspecific term and have numerous causes- ie, bacterial hormonal, viral, yeast, inflammatory, autoimmune, neoplastic causes. Salt water or baking soda rinses may ease the pain but any ulceration that stays more than 2 weeks should be evaluated by a medical or dental professional.

Canker Sore?: If it is a canker sore and occur frequently, the use of a dental soft tissue laser (aimed at the sore, without actually touching the area) has been shown to work quite well. Canker sores are from a virus which is always in your "system" and typically comes out during stressful times or due to abrasion or trauma. Try a toothpaste without sodium lauryl sulfate, if they occur often.