"That is certainly National's view ... but in order to achieve a majority in this house you have to work with other parties.
"And so when we have a byelection we don't know exactly what the result will be, we may find ourselves working with different people who have a different view.
"But it is certainly the government's position - Northland needs the bridges, we have made that undertaking and we will follow through on it."
After question time Economic Development Minister Steven Joyce reaffirmed National's commitment to upgrade the bridges no matter what the byelection outcome was.
Polls have Mr Peters and Mr Osborne neck-and-neck ahead of Saturday's voting.
The winner will replace National MP Mike Sabin, who resigned for personal reasons.
If Northland loses it will need the support of two of its three support parties - the Maori Party, United Future and ACT - to pass any future laws.
The 10 bridges to be upgraded 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.
It is expected to cost between $32 million and $69 million.
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