Why can't Australia/New Zealand flights depart from Australian domestic terminals?

Does anyone out there no why Australia/New Zealand flights can not depart from Australian domestic terminals.I am aware of multiple government hearings on the mater and as usual nothing changes.This would reduce int terminal congestion,free up gates,int slots,and immigration/boarder security could be set up thru existing treaties.We all should be able to do SYD/AKL as easy as SYD/MEL.

The Australian airports have resisted the change as they will lose revenue from international passengers.  The NZ airports have agreed to this though.  This was promised by K. Rudd in 2008. Will probably not happen!

How would Immigration/Customs be managed?

There are plenty of non-AU/NZ citizens travelling on Tasman flights.

I don't see why Australia and NZ should be classified as international flights. The climate is the same, there are hardly any biohazards that are in NZ so Customs should not be an issue. And immigration should not be a problem either as you can travel freely between the two counties. 

Not for non-Aus/NZ passport holders.

" hardly any biohazards" ... One biohazard (or axe murderer for that matter) from abroad is too many. That's why we have Customs and Immigration. 

The border arrangement between Australia and New Zealand would, what one would describe to be "Schengen-lite".

When two land masses are separated by a body of water, the psychology of the polity is affected; quite unlike the United States and Canada.

Britain and the EU is a case in point, although Europhiles in Whitehall have bent over backwards in the past decades to contort their national psyche in a desperate need to find meaning in a post Empire world.

As stated above, this was promised by Rudd in 2008, but like many other promises, unfortunately went nowhere. I remember reading there were some bureaucratic issues, so the idea was shelved. That decision seems to have been buried and of course got a lot less coverage than the original annoucement that it would be done by 2010?

If only Ozzie and Kiwis will fly between Australia and New Zealand, than it would be no brainer. But plenty of other nations use this route and for most having Aus visa has nothing to do to NZ one