I would be planting 3 lovely big trees (one family member’s family reportedly don’t want a memorial) down the back in Corroboree and having a plaque saying “these trees dedicated to xyz). That’s literally it.
I would not mention the accident or the deaths of why.
That way there is something for the families to visit if they so desire. I’d keep it as far away from the accident site as possible. Firstly for the same reason, the family probably do not want to go back to that spot. Secondly you don’t want a big “death happened here” reminded, regardless of if it’s a big or small plaque, etc.
I think the memorial should be as subtle and discreet as possible. It’s about about acknowledging the people for the family of the family so wants so.
Anything more prominent or public starts to enter the world of being for the park, and almost using the deaths as goodwill by implying “look how great we are, we care, here’s a plaque.”
I think subtlety is key.
I’d also say don’t put it in a prominent area that is likely to see development because if you have to move it, that will just cause more outrage.