As a hobbyist, then typically, the Australian Consumer Law will not apply to your transactions.
The question is not "are you operating for profit",
so much as it is "are you engaged in trade of commerce" (even if no profit).
As a hobbyist, the presumption is that you are not engaged in trade or commerce.
That being said, even if the ACL doesn't,
the overall law of contract certainly applies.
This means, basically, that you (both buyer and seller) are obliged
to deal fairly and honestly with each other in all respects.
Which brings me to this...
The term "holding deposit" has no meaning.
You're either selling the dog, or you're not.
If you take an order, then you take an order.
Even if delivery is at some future time, and even payment is not in full at the time of order.
You are not required to take the animal off the market while a customer pfaffs around.
Do not think of a holding deposit as"Don't sell it to anyone else".
Think of it as "Sold to <whoever>, to be delivered when ready".