He had once come across a head-on crash on the Kaeo Bridge.
"Luckily no one was seriously injured but there have been a number of serious crashes on the bridge over the years and it really is a ticking timebomb," he said.
Transport Minister Simon Bridges said the programme would cost $32-69 million over six years and was a logical next step from the Puhoi to Wellsford Road of National Significance.
It was essential that Northland was ready for the influx of visitors and business the motorway extension would bring, he said. Most of the funding will come from the National Land Transport Fund.
Mr Osborne's main rival for the seat, NZ First leader Winston Peters, called the bridge pledge an election bribe, while Northland Regional councillor Joe Carr said the Matakohe bridges were to have been replaced some years ago - but the project was canned by National in 2012 to meet the "insatiable cash demands of its gold-plated Road of National Significance projects".
Mr Carr, who is contesting the by-election for the Focus NZ party, said Northland had just received its first lesson on what happened if National had to work to retain the seat.
Focus NZ supported the replacement of dangerous, flood-prone bridges, but several of the region's most vulnerable highway bridges were missing from the list.
They included Three Bridges and Taumarere Bridge at Kawakawa and the bridge at the bottom of Turntable Hill, Moerewa, all of which were regularly closed by flooding.
Former MP Mike Sabin campaigned hard to replace the two one-lane bridges at Matakohe without success.
The bridges up for replacement are:
SH1: Waihou River (Rangiahua) Bridge
SH10: Taipa Bridge, Kaeo River Bridge
SH11: Tirohanga Stream Bridge
SH12: Taheke Bridge, Waimamaku River (Hallahans) Bridge, Waimamaku River (Lowes) Bridge, Darby and Joan Kauri Bridge, Matakohe River (Hardies) Bridge, Andersons Bridge.