I concur that natural stones typically provide bitier edges at higher grits, and likewise agree with many of the proffered options (Big fan of Dan's Arks, and also Belgian stones in general.). However, I shudder to think the shipping cost of getting a Dan's Arkansas stone of any persuasion (The only brand I would recommend... Most others aren't lapped properly, and an improperly lapped Ark is a nightmare.) to Australia... The prices of BBW's & Coti's appears to be very high in Australia, as well:
Belgischer Brocken Blue. Belgian Ardennes whetstone with a grit of 5000 .
If you're going to Japan, looking for a J-Nat would probably be a more economical option if you want to go down the natural stone road. I am not familiar with the process of retail shopping for such a thing in Japan, but there do seem to be some good brick & mortar stone and knife shops, and you could probably get some help from one of them to select a stone. Even my cheapest little Aiiwatani Kiita koppa Lv. 3-3.5 tends to create edges that last a bit longer, and have a bit more bite (Caveat: on suitable steels.) than 6-8K synths. I'd say looking for a Lv. 2.5-3 finisher would work fine, and jump up from an NP 3K finish just fine.
My advice, if you want to try a less expensive solution, would be to pick up some 1-Micron diamond paste/spray on a hard balsa or similar strop to follow up the SP 2K or NP 3K you already have. A big jump like this creates a so called 'hybrid' edge, polishing the existing teeth a bit more, and amping up the sharpness while still preserving some bite. This technique also tends to work on a very wide variety of knives in different steels/HRC's, whereas some natural stones can be a bit more 'specific' in how they respond to different steels/heat treatments.
Hope this helps.
