Working in Australia - six months with one employer
If you hold a Working Holiday (subclass 417) or Work and Holiday (subclass 462) visa, the main purpose of your visit must be for an extended holiday. You can do any kind of work over the course of your 12 month stay in Australia, however the conditions of your visa limit you to a maximum period of six months work with any one employer, unless you've been given permission by the Department of Immigration and Border Protection to work longer.
First Working Holiday visa:
No, a Student Visa can not be "converted" into a WHV within Australia:
You must be outside Australia when you apply for your first Working Holiday visa and when the visa is decided.