The first thing to do is, relax! Blue-tongue lizards are not poisonous and do not pose any threat to people or their pets. In fact, they can be a very helpful friend to have around the garden as they eat snails and caterpillars and other insects and can help keep the population of these invertebrate garden pests down.
Blue-tongue lizards are slow moving and are not aggressive. They live under rocks or in crevices and come out during the day to sunbake and look for prey. If they are threatened or cornered they open their mouth wide and stick out their tongue to frighten off potential predators. This might look scary but they will not bite unless they are attacked or you try to pick them up and hurt them.
You can read all about blue-tongue lizards in your garden at this helpful NSW government site:
The Australian Museum also has a fact sheet on blue-tongues: