Hi Luke
You are living in a very tightly controlled strata
I assume due to a very controlling person on the EC or them having numerous problems in the past that they have tightened the rules. I agree with Peter C they laws conflict that they are denying you use and enjoyment of the common property. Have they given you a specific reason for the knock back part from you ‘may’ be too noisy. It may be worth setting out a time frame for the gathering and submitting that.
As for the fine. They can enforce fines (in WA at least) if you breach a by-law. under s1031 of the Act, however the fine can not be more than $500.
Alternatively rather than jumping through hoops it might pay to have the gathering at the beach or park?