The Northland seat has been held by National for at least three decades.
It is shaping up to be a close race between Mr Osborne and NZ First leader Winston Peters, who are neck and neck in the polls.
The National Party yesterday promised to upgrade 10 one-way bridges in Northland as part of its campaign.
Mr Osborne and Transport Minister Simon Bridges announced the six-year programme, expected to cost between $32 million and $69 million.
The 10 bridges are all on state highways 10, 11 and 12 and include two bridges near Matakohe, one in Taipa, two on the Waimamaku River and the Kaeo bridge.
The timing of the announcement is likely to raise eyebrows. Mr Peters last week used the bridges as an example of the neglect the region faced under National.
Mr Osborne said he had been campaigning on the bridges since being elected, and described the Kaeo bridge as "a ticking time bomb" after a number of serious crashes.