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Welcome to the Bunnings Workshop community @MarianneW. It's wonderful to have you join us, and many thanks for your question about vent pipes.
You would need to follow the pipe up to see where it terminates. If the pipe you have circled in yellow travels up and has a vent outlet at the top, then it's entirely likely that it is a sewer vent pipe.
Please let me know if you have any questions.
Mitchell
