Longreach, Merry Xmas to an "OZ" on the east coast.
I followed what seems to be a family tradition, as my father shot his first fox at age 5, and had me shooting at age 7. My eldest son by the age of 9 was confidently and competently shooting the 17/222 Mag imp, 224 Clark, 25/06 AI, but seemed reluctant to try the 7mm Rem Mag, and I was comfortable with that.
The Eastern Grey Buck Roos that my son shot that night were the biggest of that species I had seen, and for some strange reason all seemed to be confined to an area of about 500 acres. Since then I have occasionally seen the odd thumper that would be around 7 feet, but the majority of males would be 5.0 - 6.0 feet, with the females being considerably smaller.
I have not seen a 257 Weatherby Accumark, to be able to pass judgement. However, I have always believed that for the price of a Weatherby, I could build up a better rifle from quality custom components, and still believe that would be true.
"Hired gun's rig" certainly looks good, and he gave a good description of the level of performance from the 257 Weatherby case, and it certainly would handle about 90% of Australian game.
The cost of 257 Weatherby loaded ammunition is expensive, and if you want to reload, the Norma unprimed cases at $A170.00 per 100, are over 3 times the price of Winchester or Remington 25/06 cases. This may or may not be an issue for you to consider.
Although I do shoot various other calibres, I believe that the .257 calibre when using cases of about 25/06 size capacity (65 grains) or larger, combined with the 100 - 120 grain bullets are the best option when used on Australian medium game. The 270Win seems to work best with the 130 - 150 grain bullet weights, and I agree with you, that they are more than is needed for about 90% of your shooting.
The 25/06 will do a good job, and is the cheapest option, while the 257 Weatherby is probably the most expensive, but will have the ability to deliver the extra performance when the need arises. Good luck in whatever you choose, and email me if you want some reloading data for the 25/06 or 25/06 AI. Regards, Brian.