Hi @Popeye,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
From the way you’ve described it, the main issue isn’t just a routine blockage. The presence of sand in your waste pipe suggests either a cracked section of pipe underground or sediment that has washed in from outside during heavy rain. Sand is particularly difficult to shift because it settles and compacts, which is why your drain snake attachment is struggling. Even if you manage to push some of it through, chances are it will build up again unless the underlying cause is fixed.
The overflow relief gully is the right place to focus your efforts, as it’s designed to act as an access point into the sewer line. You could try clearing as much loose sand as possible by hand or with a wet/dry vacuum before attempting to flush the line, but a small pressure washer or snake simply won’t have enough force to deal with compacted sand. In most cases like this, a plumber would use a high-pressure jetter or mechanical eel to break up the blockage, followed by a drain camera inspection to confirm whether the pipe itself has been damaged.
Given the type of blockage you’re describing, the most reliable next step is to call a licensed plumber. They’ll be able to clear the sand properly, check the condition of your pipes, and prevent the problem from recurring. While a bit of DIY effort might buy some temporary relief, the combination of gunk and sand in a main waste pipe is usually a sign that professional equipment and expertise are needed to resolve it for good.
Let me know what you think and if you have any further questions, please don't hesitate to ask.
Jacob
