If it's photoshopped it's pretty convincing, and if it's a big rubber crocodile then... who has a big rubber crocodile anyway?
No crocs in Brisbane. Every year is the same claims "I saw a croc!"
A brown snake in the water will climb in a house to take refuge, not to attack. they're just trying to get out of the water, just like cockroaches piling up on traffic lights.
Some sea snakes (at least in Australia) are venomous. Also, snakes (including venomous ones that might normally live out of the water) can swim and in flooded regions would be forced to do just that. The red bellied black snake isn't as dangerous as the inland taipan or eastern brown, but it does like water.
Some of the most dangerous snakes in Australia always live in and near watercourses, and are usually disturbed while sunning themselves near a river when someone decides to go for a swim. The types of snakes found swimming around in the Queensland floods are not the types you want to be messing with.
Try this one on for size : http://www.snakecatchers.com.au/Red_Bellied_Black_Snake.html
I saw a 3m one of these about a week ago sunning itself next to the river near my house. And I don't live in the bush.