Pretty sure (like 75%) that its automatic. Even if it isn't, it doesn't usually provide a huge boost since there are so many schools out there that are technically underrepresented (and all things considered, there are much worse/impacting SEAS factors than that one).
Actually (and sorry Edward21 if you didn't do this) you had to fill out the details of section 1 on the SEAS application - basic details like gender, DoB, school etc. Apparently they use this info to check if you're eligible for stuff like under-represented school... so if you didn't do that section... sorry
http://www.vtac.edu.au/who/seas/categories.html* By submitting category 1, you are giving permission for the following information to be passed onto institutions to which you have applied:...
Under-represented school-to identify if you attend one of various under-represented schools identified by institutions under this category-also refer to Category 6 if you are applying to La Trobe University, Federation University Australia or Victoria University.