These are WD's helium filled 10TB SATA 3.5 HDDs intended for NAS use. NAS use because of the low heat/power facilitated by the helium filling, the 5400rpm and the internal block read optimisation. But of course you can use them in computers too, they're ideal for archiving, home servers and media serving.
I'd been looking for them in Australia, the best price I found was $629 at MSY. Then, no doubt thanks to Google spying on me, I saw an ad for them at US$379 (AU$477) at B&H Photo with a reasonable shipping charge of AU$20ish for 10-15 day delivery (you can pay a little more for 5-7 days). So I ordered two.
Reviews say nice things about performance and power usage, for spinning metal anyway.
More info here and review here.
You can also get these at Newegg AU for AU$481 plus shipping.
B&H also have the Pro version (7200 rpm) of this drive for for US$499.
https://www.ozbargain.com.au/wiki/help:first_post