Does anyone know how to identify snakes (worried for my little dog)?

Saw a snake curled up in the rafters of my undercover outdoor entertainment area thingy. I reckon it was at least 1.5m long. It was very dark in colour on top with a lot of yellowy spots/pattern. It was out at twilight. Its underside was a yellow/cream colour. Is it just a python, or something venomous? How do I tell? In South_east Qld (snake removal guy wanted over $150 to catch it... but wasn't out in time)