Regarding the SNOWY links to the NSW and VIC NEM regions, the limits are explained on a regular basis by the NEMMCO MT PASA (medium term supply predictions) Interconnector quarterly reports. The March 2008 description was (from http://www.nemmco.com.au/psplanning/200-0146.pdf ) was:

The limits used in MTPASA for the Snowy – New South Wales interconnector are:

The limit for flows from Snowy to New South Wales varies between 3500 MW in winter and 2800 MW in summer and is dependent upon line ratings, Snowy generation profile and the magnitude of loads in southwest NSW.

The limit for flows from New South Wales to Snowy is determined by thermal and transient stability limits. This limit is highly dependent on loads in southwest New South Wales. When the transfer from Snowy into Victoria reaches 900 MW TransGrid arms an emergency control scheme to allow higher transfers. The limit for flow from Snowy to New South Wales in MTPASA is largely limited by the restriction that no more than 1946 MW is supplied to New South Wales from neighbouring regions via interconnectors. This restriction is modelled to ensure that the assumptions used to develop minimum reserve levels for New South Wales are maintained in MTPASA. TransGrid has advised NEMMCO that due to the implementation of an automated control scheme, provided by Snowy Hydro Limited, short-time ratings on transmission circuits north of Snowy can be utilised at certain times. When the short-time ratings are available the transfer limit on the SNOWY1 interconnector will be increased by up to 200MW in the Snowy to NSW direction. (See also NEMMCO Communication No. 2356).

The limits used in MTPASA for the Victoria - Snowy interconnector are:

The limit for flows from Victoria to Snowy is determined by thermal and transient stability limits. The thermal limit is due to the rating of the South Morang F2 transformer. The transient stability limit reduces as the Victoria region demand increases. The limit for flows from Snowy to Victoria is around 1900 MW. On hot summer days this limit will reduce below 1900 MW unless a load shedding Network Control Ancillary Service (NCAS) is enabled to allow use of the 5-minute thermal rating of the Dederang – Murray 330 kV lines. Without NCAS, the limit reduces to 1700 MW at 35 o C and 1600 MW at 40 o C. The National Electricity Code clause 3.11.3 (b) requires NEMMCO to monitor whether it is economic to enable NCAS services to increase interconnector capability. As MTPASA is a reliability assessment the NCAS is assumed to be available. With the NCAS service enabled the Snowy to Victoria limit is 1900 MW at 40 o C. The upper limit is determined by voltage stability considerations and is typically 1900 MW in summer.

How much would an interconnect to NZ cost (using the Basslink cost as an indicator) ?

Links connecting the various continents have been part of long term visions for years (the most prominent example being Bucky Fuller's GENI proposal).

Why link to NZ? 2000km is a very long distance for an undersea cable and NZ has a near shortage of power most of the time. You could build plenty of variable wind power over there without need for importing it from Australia.

The East-West link may be a bit too pricey too but the slightly lower sunshine hours in Western Queensland would make it viable to have the Queensland to South Australia link in place. Maybe a combination of solar and geothermal in that region?