What is this in our garden?

Hi,

Just bought a standalone house in Altona, Vic few weeks ago. I found this in my garden. Thought it was for decoration in the first place but it seems not when i looked closer. Anyone knows what it is? Is it kind of sewage pipe marker or something i should be aware of. I planned to plant a tree nearby so i thought i’d better to ask first

P/s: not sure if this is right subreddit to ask this. Happy to remove it if not.