I've owned and used a 6.5 X 55 carl G for better than 20 years.
I know what it is capable of.
I shot a rougue buff 30 years ago with a 243 using 85gn bthp projectiles.
Mind you I shot it from a vehicle so had plenty of protection just in case things went wrong.

Buff have been shot with 22rimfires,22hornets ,222rem,243,ect ect

Therefor in your opinion MICKEY the above are suitable for Aussie buff.

JOHN,the recomended mimimum weight & calibre for buff is 375 cal and 250 gn projectiles.
A 270 falls a long way short of that.

I would like to see your mate out on the flood plain,no backup with a badly shot buff on the prod,armed with his trusty 270 buffalo rifle.
The dingo's would probably clean up what was left.

I've shot 15 buff,hundreds of scrub bulls,100's of tons of donkey ,horse,camel,goats ect ect.

YOUR QUESTION SHOULD HAVE BEEN:-
IS A 6.5 X 54 considered suitable for water buffalo ?

" NO IT IS NOT"