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Welcome to Just Answer, I will be assisting with your question today. Please bear with me if I need to ask you some clarifying questions. Thank you!
As I understand from the information stated above, you already have a visa in your passport, do you?
Did you get a family permit from outside the UK?
I see, now the thing is that there is no such a thing as activating your visa but in the future, if you apply for settlement, etc this whole thing may produce confusion.
If it is possible, I would leave the country and enter the UK with my family permit Australian passport.
It is very important to enter the UK with your family permit and apply for pre-settled status as soon as possible. You do not want to lose the status.
This is not correct after the visa is expired you will lose your status.
You usually need to apply within 3 months of arriving in the UK or before your family permit expires.
I hope I have helped! Thank you
You have to wait for the deicision.
When you apply, you can call the settlements team, and once you have your case in the system, you can ask them if you can leave while waiting for the decision.