You can only sponsor your girlfriend for a partner visa (which includes spouse, prospective marriage, de-facto) if you are a permanent resident. As I understand it I believe a 475 is provisional (temporary), so you wouldn't be able to do it.
The Partner Visa pamphlet says
You may not be able to sponsor your partner if you:
• are not an Australian citizen, Australian permanent resident or eligible New Zealand
• have previously sponsored 2 other partners for migration to Australia (includes
sponsorships you may have withdrawn where your former partner obtained permanent
residence on family violence grounds); or
• have sponsored another partner within the last 5 years; or
• were sponsored as a partner yourself within the last 5 years; or
• are the holder of a Woman at Risk visa (subclass 204).
You may still be permitted to sponsor your partner in compelling circumstances,
• if your previous partner has died or abandoned the relationship leaving young
• if your relationship with your current partner is long standing; or
• if you and your current partner have children of your relationship"
Have a look on http://www.immi.gov.au/migrants