Note that the swingbridge at Smyth Hut has been destroyed (2011). Some parties report crossing 300m above the hut but the nearest viable crossing when I visited (Feb 2016) was at the Vane Evans forks
A good route follows the northern bank of the Wanganui upstream from the Smyth confluence to the Vane / Evans forks - mainly on river rock with a few boulder scrambles.
The Evans was not crossable at the forks when I visited - the first good (solo) crossing was 800m up river just above the large waterfall which descends the southern valleyside. The Vane is generally crossable at the confluence unless there's been rain or in high meltwater conditions (early morning is better).