The stage is expected to take until the end of next month to complete, meaning the path will be closed until then.
People are encouraged to use a detour via Heads Rd in the meantime.
"We appreciate that this work will cause disruption to pathway users. Please take care on Heads Rd during the construction period."
The first section of shared pathway from the Whanganui City Bridge to Bedford Ave opened in December 2016.
Once complete, the $1.29 million pathway will create a riverside trail that connects the town centre to the Tasman Sea and is an iconic end point for the Mountains to Sea New Zealand Cycle Trail.