Meteorology is inexact, particularly in a weather situation like NZ lies in.
The Tasman is dangerous. Because a passage would take about 9-10 days and the weather sytems prevail for about 6 you must expect a storm with a long fetch and therefore big swell.
On that particular day I would be surprised if that were the correct actual forecast. Even if it were, just looking at the weather map would have made me sceptical though I am not a meteorologist.
Forecasts change, a weather map is more helpful particularly for a long voyage.
What it looks like happened is the map forecast for midday Thurs was quite benign, but very bad as forecast for midnight, as a tropical low off the coast intensified and then joined a major low below the South Island.
If they checked out with Customs at 8 am that would be by appointment made the previous day on the basis of a decision made on Wednesday.
I think the forecast changed on the Wednesday night or Thursday at some stage just judging from the am paper map.
It would have been prudent to check. They may have been lulled into a sense of security by initially being somewhat sheltered by the land.
Once they were given clearance they cannot stop except in an emergency if approval is given. They could seek approval to turn back.
The conditions were nasty. When the boat should be fully secured is debatable. Some might say when the passage begins, or when bad weather is expected.
The winds would be SW, the shore lee, and the course beam on to a heavy swell. Not a good mix.