I am from NZ and have spent 90 days in the USA with an ESTA visa waiver, and I am going to Canada before returning to NZ. My return flight is from LA but I have already used my 90 days of my ESTA and have not technically 'left' North America (Canada apparently doesn't count). My question is, how do I catch my return flight from LA? Can I get another ESTA even though I haven't left North America?
From this related question - Do I need a US visa to transit (or layover) through an American airport?
Unlike many other countries, US airport do not have any form on physical immigration controls when you are departing the country on an international flight. In fact, in most airports there isn't even a concept of an "International" terminal/gate, with the same gates frequently being used for international flights and domestic flights.
As a result of this they can't enforce the concept of a 'transit' passenger - once you're in the departure area, even if you got there on the pretext of catching another international flight out of the country a few hours later, there's nothing to stop you boarding a different domestic flight, or even simply walking out of the airport!
As you have already used up the days granted under the VWP, you will not be granted entry any more under the VWP program (until you return to your country of residence). Don't bother trying to 'get another ESTA' - that will just effectively renew the ESTA (i.e. the pre-authorization that will likely, but not guarantee, entry under the VWP program), but it won't affect the current conditions of your stay, which is governed by the VWP program.
If you're leaving Canada for the US by plane, you will be pre-cleared in Canada, and therefore you won't even be able to board the plane to the US, even if you manage to convince the airline staff to let you check-in. If you're leaving by road, you'll probably be knocked back at the border. I assume something similar will happen by sea as well.
Consider getting a transit visa (http://travel.state.gov/content/visas/english/other/transit.html) to permit you to transit through the US to get home. It will take time and (a bit of) money though!
Alternatively, assuming you're flying Air NZ, you could try switching your LAX-AKL return flight to YVR-AKL, which will allow you to bypass the US altogether.