Assuming they are staying in the Central City
Go down to the railway station. Get on a train to Upper Hutt. They go every half hour. Get off at Woburn station. Then about a 5-10 minute walk to the ground either with a map or follow the crowd. Dare say there will be a few on every train going to the match on the day. Ride will take about 15-20 minutes. Cost for adults $5.50
Except that, as previously noted (see the post by Warrick Hunt quoting the "Buses replacing trains timetable screenshot" near the bottom of page 23 of this thread), currently the Tranz Metro website says that ALL Hutt Valley trains are cancelled on Feb 14th, and are being replaced by buses, frequency etc. TBA.