Hi @billy3,
Thank you for your question and welcome to the Bunnings Workshop community, it is brilliant to have you with us.
The installation instructions on a Everhard Industries Series 600 Stormwater Pit which I believe will be similar to yours, state that they should be installed on a compacted base of drainage gravel, then concrete should be poured around the pipe entries to ensure the pipes are adequately supported. Then the excavation around the pit should be backfilled with a suitable compacted material before a concrete "collar" is poured around the top rim of the drainage pit. With a 600mm stormwater pit, they advise that the concrete collar is no less than 150mm wide by 150mm deep.
Let me know if you have further questions, I am more than happy to assist.
Jacob
