ACT has refused to disclose its membership numbers to the electoral commission, even in confidence, out of fear that the numbers "will be leaked to opponents". While they're not legally obliged to disclose the figure, party membership is one of the criteria on which broadcasting allocations must be judged - and failing to disclose it may result in a lower allocation. But it may also result in a higher allocation, as the Commission may not want to risk underestimating - and given the contempt ACT has previously shown for election registration (they have previously outright refused to disclose funding or file post-election returns), I'm more than a little suspicious that they're trying to game the system in this way.
In the absence of proper disclosure, I'd suggest that the Commission go on the sole membership figure it has available: ACT's declaration that it has at least 500 financial members, required in order for it to contest the list. And if ACT is unhappy with that, they know exactly what they can do about it.