A deal was posted this morning for this external hard drive/NAS. OP originally thought it came with a 6TB HDD but commenters pointed out that the description said "Install your own compatible hard drives up to 6TB (drives excluded)" and decided it was just the shell. I'm not so sure.
The model number is WDBCTL0060HWT-AESN which is shown on the Umart website as including 6TB of storage. There's also the WD Product Data Sheet linked under the "Documents" section of the JB HiFi page that clearly shows the model WDBCTL0060HWT is 6TB along with the other models in the series, all of which have drives included. The 60 in the model number seems to correspond to the 6TB drive size. Lets not forget that the title is also "WD My Cloud 6TB Capacity NAS (White)"
All in all I'd take the bet that if you order this product you'll get one with a 6TB drive included.
Go into a JB and ask them to open one up?