The bit that is a bit suss to me is this:
Maybe I am reading your post wrong but they most definitely do ask about loans and I always disclose my LOC (limit and owing) as a mortgage like I would for any property. I know full well it's listed as a loan account on my credit report as I've pulled all 3 of them and in general, my advice is to submit your applications exactly as it appears in your credit file, my only deviation tends to be that I give a more recent balance.
A lender who sees a loan for $300K on your file which isn't disclosed is going to reject your application, imho. A lender who sees a $300K loan with $0 owing is unlikely to assume you'll just draw down the entire amount tomorrow, or those who have made advance payments to their mortgage and have a sizeable redraw would be in a similar situation. They will understand that you have the potential to draw down on it and maybe they'd even model a certain % of drawdown when calculating serviceability, I don't know, but if you don't list it at all that alone could be justification for a rejection.