At the quarry on Whangaehu Valley Rd (E2720025 N6182735) park up and descend an old creek (steep) on the upstream side. Pick your way down through rocks and cutty grass then across a muddy swamp and you are at the river's edge, phew! You can leave a car at the Te Tui farm entrance (E2719770 N6177085) or do a longer trip right down to Collier's Bridge. Allow 2-3 hours to Te Tui farm take-out and expect an hour of uphill boat carry.
The river is continuous Class III at most levels with 2 new rapids that should be scouted. These have formed since the recent lahar came through. The first is a river-wide feature which has a sticky hole on river right at higher flows (boof on river left) E2717940 N6181580). This is about 1/3 of the way down. Later on 2/3 of the way down and after a right hand bend is a very steep drop with a rocky outflow which will cause damage to anyone upside down etc. Scout this from river right and/or run a less spectacular line on hard left (E2718730 N6179420).
Make a point of bringing your own drinking water as the W originates from the Crater lake on Ruapehu and is naturally acidic as well as passing by the Karioi pulp mill. Enough said.
The Whangaehu is great for new Class III paddlers with lots of rapids but nothing too intimidating apart from the 2 rapids mentioned above. In November 2007 there were a number of trees and logs in the river - probably left over from the lahar.
If you are fit and have plenty of time it is best to run the Whangaehu from the Quarry right down to Collier's Bridge rather than taking out at Te Tui. This will avoid the big slog out at Te Tui. If you have less experienced paddlers then they can put-in at Te Tui as the lower run is easier with less frequent rapids (see Te Tui to Collier's Bridge)