Try your local scrap metal merchants.

In answer to your thread title question, I have to say pretty much any place after a cyclone!!

But yeah, like Jode says All Yacht Spars in Brisbane should have the juice you are looking for but two 6 metre lengths freighted to you in NSW might cancel any cost benefit.

www.tempospars.com.au/masts.html

Maybe their TS104 is what you are looking for? Not far from you at Brookvale, so worth a phone call for prices of pieces.

As you are in NSW I would try Whale Spars

www.whalespars.com/

They are decent people and will most likely help you with your project.
Ed

Thanks guys I think I found the right supplier in the list you gave me.

The project is a swim platform/trampoline for the back of the cat. Last season the experience of getting out of the water wasn't great for kids and grandparents so I'm trying to make it more pleasant, while also giving people a place to rest when swimming. It also needs to be easily raised out of the water to form a net/pushpit. I get very nervous when kids or pets go near the stern when underway so it will have two jobs.

Using mast/spar section is convenient for strength/corrosion/weight but also means the net that forms the centre of the hammock will attach in the channel. This will be more comfortable for the backs of knees and soles of feet. Hope that makes sense.

You mean like the one on the back of Aristocat2 ?
perfectnaturecruises.com.au/galleries/