Get professional advice as you appear to be confused about the rules. With a diploma you can get an buttessment in a 40 point occupation, which could be enough for the 882 designated area sponsored.
Some people with a 40 point occupation can still get to 110 points for send sponsored but you'd need to look at the offshore equivalent of 881 (138) and you'd have the additional complication of the Sydney skill list rules if your sponsor lived in or near Sydney.
In limited circumstances, a person with a 40 point occupation can also qualify for the SIR visa if there's no way to get send sponsored. Also, RSMS may be an option for some.
Rules for onshore students looking to get permanent residence are incredibly complex and in the last few years hundreds, if not thousands, of students have had to go home because they got confused and didn't make the right visa decisions.
It's important to use a competent pracbreastioner, call IARC in Sydney, they don't give this kind of advice for free but can refer you to
Jeremy
-- This is not intended to be legal or professional advice in any jurisdiction
