Sorry, this is super late, but hopefully it is still somewhat helpful. I went to LaTrobe today and spoke to someone about Aspire and listened to a presentation about it where a lot of these questions came up.
From what I was told, there is no threshold for the amount of work you do, as long as you had an impact on someone in your community. In your statements, you should focus on the way you assisted others and how you made them feel. As long it is evident you made an impact, the hours do not matter. I volunteer three hours a month with a certain charity, as well as two hours a week with another and have for a year and a half, and they told me it was certainly enough.
You are only eligible for the early offer you receive on the condition you achieve the minimum ATAR. If you do not, you are unable to accept your offer and will not be given a place.
From what they said, almost everyone who applies receives a conditional offer (although not all who receive an offer go on to achieve the ATAR). Those who do not receive an offer usually did not write an eloquent enough piece or explain the impact of their work well enough.