Husband and I have been living in our current (rental) place now for 6 months - we are still getting a lot of post for the previous occupants which we just keep sending back as not at this address. However in late November we got home after to find a large parcel sat in the carport. It was a robotic pool cleaner - interestingly there is no space for a pool at this place at all.

The parcel was delivered to the correct address, but addressed to someone that we have never seen on post before. We did the decent thing and bought it into the house for safekeeping, with the intent to contact the delivery company. Before we could do this (we had a lot going on) we got home to a business card tucked in the front door, with a note saying that the guy had a delivery incorrectly sent to our address and could we ring him so he could collect it. We have now rung him on several occasions and even sent texts messages saying please contact us to arrange a suitable time to collect his parcel. The last time was the text this week, in which we said that if he hadn't contacted us by end of the month we would be getting rid of the parcel - yes i am sick of it taking up space in my house.
What are our obligations, considering he hasn't got back to us, even when we have tried a number of times?

Return to Sender by registered post COD?

Not sure how things work in OZ, but in the UK, if the sender hasn't claimed the item back within a specific time frame, then the item becomes yours by default, we used to keep all those unasked for charity Christmas cards, and use them the next Christmas. The onus is on the sender, especially if you have had contact. I would suggest contacting the company, and saying if it isn't collected by, and give them 3 or 4 weeks, then the item will be sold on Ebay.

I suppose blowing it up isn't an option?

Not sure what COD is but the box is a close to 1m×1m×1m large.

We will be sending an email to the sender (company) along with a final email to the intended reciepent otherwise will be giving it as a late present to friends with a pool.

It has cluttered my house up for a month now and I am sick of the sight of it now.

I think someone has to store something for 3 or 6 months before they can dispose of it. (Maybe ring a storage company to confirm?).

COD is cash on delivery. So, you post that big package back to wherever it came from... with them to pay the freight on receipt. (Not sure you can register and COD actually) but either way, you send it back to whomever sent it. That's what I'd do. Or find the local outlet from whence it came and take it there.

If you've only had it a month and give it to someone I'd be concerned you'll be liable if they come looking after silly season is over.