Labour leader Andrew Little says the Prime Minister should sack Transport Minister Simon Bridges over claims Mr Bridges broke ministerial rules in developing the Northland byelection bridges pledge.

Mr Little made the call today, saying Mr Bridges sought advice from the NZ Transport Agency prior to announcing plans to upgrade 10 one-lane bridges in Northland during the byelection campaign.

A rule in the Cabinet Manual states that requests from officials during election campaigns must only be for the purposes of their portfolio areas "and not for party political purposes".

Mr Little said the guideline existed so that the incumbent party did not have an undue advantage over political rivals.

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"A Cabinet minister, Simon Bridges, sought advice from a Government department at the time of an election in relation to a pledge they were making in that election.

"He has breached the Cabinet manual and the Prime Minister, if he has any standards at all, has to take action against Simon Bridges."

Mr Little said he would sack a minister in a similar situation.

The Cabinet Manual is a set of guidelines of behaviour for ministers over which the Prime Minister has authority. Mr Key said he was not aware of any rules preventing ministers getting advice from officials to develop policies.

"During election campaigns, and a byelection is no different, you see political parties making all kinds of promises."

He said it was appropriate. "You're allowed to use resources of the officials in terms of what would be Government policy."

Mr Key said he had not seen any evidence to support the claim Mr Bridges breached the manual.

"The Government has the authority to make decisions around where it wants to invest in infrastructure."

Mr Bridges was not available for comment immediately.

In a letter to Labour advisor Rob Salmond, Mr Bridges stated that he did not seek any advice from officials on the commitment to upgrade the 10 bridges, but had received "general information" on the one lane bridges in Northland over a period of time.

Documents released to Mr Salmond under the Official Information Act showed that included emails with information and approximate costings just days before Mr Bridges announced the 10-bridges plan in his capacity as National's transport spokesman during the byelection.

Public servants are also subject to tight rules about fulfilling requests for policy costings from ministers during an election.

The State Services Commission guidelines state government departments can only provide costings if the request comes from the Finance Minister or the relevant portfolio minister.

Departments must also notify the Finance Minister's office of any such requests.