I personally worked in Audit/ Assurance at PwC, and it is very possible to do an international secondment. I have lots of friends in the other Big3, and I believe the opportunities are just as possible there.
At PwC, in most cases, you are required to finish your 3 year training contract within Audit – i.e. complete your ACA, before you go on secondment. I would recommend you show your interest early on though with your people manager (basically your career counsellor) so they know your intentions. There is a lot of movement, especially within the Banking sector, between London, New York and Hong Kong. However, there are literally PwC offices everywhere and if you make your business case strong enough you have the opportunity to work at any of them!
I hope this helps! Good luck.