The advantage of going to the Westcoast on Labour weekend is that we have a 3 day weekend so a smaller proportion of time would be spent traveling.
Kaikouria is a tolerable day trip from both directions.
Blenheim > Westport: 220km
Blenheim > Kaikouria: 110km
Christchurch > Westport: 300km
Christchurch > Kaikouria: 150km
Nelson > Wesport: 175km
Nelson > Kaikouria: 210km
(+ or – 85km for Greymouth)