Why has SYD held up so well in the current economic enviorment?, page-12

SYD is a stapled security and a lot of the rough and ready information that is available from share trading platforms and websites don't always treat this the best way. You really need to use their annual reports. What a pain!

Roughly, when you buy SYD, you get SAL (which is basically the airport and operations) and SAT1 (which lends money to SAL). So a significant part of SAL's debt is owed to SAT1. Or put together, a significant part of SYD's debt is owed to itself.

They do this to avoid tax, each year SAL pays interest to SAT1, and SAT1 pays that interest out to SYD holders as part of the SYD dividend. This tax avoidance arrangement therefore increases the value of the airport to international investors and, arguably via the additional demand, domestic holders. (It also explains why you don't get franking credits, if you were wondering).

But this means in calculating SYD's debt to equity, you need to make certain that your data contains the sum of both entities, SAL and SAT1, but also that you need to cancel out the debt that SAL owes to SAT1, with the asset the SAT1 is owed from SAL.

After doing this it makes things look better. The book value isn't negative, together they do have about a billion dollars of positive equity.
And since a SYD shareholder is also the beneficiary of a significant chunk of SYD's debt, a VERY rough calc I did without delving too deep showed me that interest bearning liabilities for the SYD holder are only about 8.5 billion. Total liabilities about 12 billion (one of those "liabilities" is a long standing deferred tax liability of $1.8 billion which it seems they may never have to pay).

This is one reason I prefer AIA to SYD: it's that you can read and understand their annual reports with about 10% of the effort.