OA and autodiscover overwritting settings with internal uri

Basics are OA is set to external oa.blah.gov.au and internal to notes.blah.gov.au.

This is due in part to internal users go through a kemp NLB and the external clients through a TMG array (server farm of the two CAs servers).

To initial setup works as expected until autodiscovery does its 15min update and sets the external OA clients msstd: to the internal uri.

Now I'm sure I saw a blog some where that mentioned how fix this but for life of me I cant seem to find it.....so any help would be great.

Thanks