Are SYD dividends sustainable?

I've been looking at recent reports for SYD and trying to understand their dividend, distribution model and whether it is sustainable long term.

From the latest half yearly report they are paying out in dividends at 15 cents an amount of $337mil which is more than double their npat $159mil for the half. I see from their presentation (page 10) they seem to be paying back the entirety of the operating receipts (so adding back depreciation and amortization expenses).

Their debt is $8.4 billion (up $250 million from previous report), cash up $110 million.

Now they do have decent growth across all segments but surely going forward there is a limit to how much growth is achievable from this single asset Sydney airport (not sure they can schedule in many more flights and there is only so much you can keep increasing landing fees and rentals on retailers - already seem very high).

So to me I question how sustainable it is to keep paying out this high dividend distribution rate. Their debt seems to be increasing and if down the track interest rates go up that could be a big problem. I would be interested in hearing the opinion of others, maybe I am missing something or don't entirely understand their business model or long term plan.