The time to clear the subtropics is by late November December so cyclones shouldn't be an issue unless you want to go via New Caledonia and are running late.
The difficulties are twofold.
The predominant winds are SW or W. The course from Nelson is 283 and from Cape Reinga 264. So while there is marginally more chance of a close reach from the South Island in both cases the chances are there will be a lot of beating.
The distances in each case are about 1150 nm making it a probable 10-11 day trip depending on the boat and course.
The difficulty is that the weather systems tend to last about 7 days. So if you start on a high where the winds could be favourable if it were not too high in the Tasman you will have unfavourable stronger winds in the last few days the seas being accentuated by the shallow Australian waters, or conversely you start with a few hard days.
The Tasman has a reputation as one of the most difficult passages. One guy I met had done it 20 times and said they got bashed everytime. Conversely some have had a charmed run.
Consequently most head a bit North to say Norfolk Island or New Caledonia for easier winds, albeit a longer distance.
You need to consult the pilot charts.
Without knowing the boat or experience of the crew it is difficult to comment much more. However the chances are that you will experience heavy weather at some point of the passage, and at the least should be well prepared and able to wait for a relatively favourable weather window.
Last edited by chris_gee; 07-04-2007 at 07:13 PM.