The Auckland City Council has admitted paying twice for some footpath renewal work, but does not know how much the error cost ratepayers.

A Herald investigation has found a flaw in the council's footpath contracts, which has resulted in overpayments to John Fillmore Contractors since 2004.

The company has been paid twice for footpath and kerb-and-channel work that overlaps.

The overlap amounts to a strip of footpath about 45cm wide that butts against the kerb. Double payments were not made where there was a grass berm between the footpath and the kerb or where there were vehicle or other crossings.

Records for the past two financial years show the contractor renewed 194km of footpaths, but the council cannot say how much was paid for twice.

The city's transport general manager, Don Munro, said yesterday that the council had been aware of the overlapping for several years, but had agreed on a rebate - believed to be about $9 a metre - only in the past few weeks.

He did not know if the council would seek repayment for previous overlaps.

"A judgment was made in the past, and it was decided that the overlap situation was not a significant issue."

Mr Munro said reasons for not correcting the anomaly until now included:

In the original contract, the overlap was for asphalt 25mm deep, rather than the more expensive 100mm-deep concrete.

Concrete had been phased in over the past few years but the incidence of overlap was not significant.

In most cases where the footpath was up against the kerb, the kerb did not require work and therefore there was no issue.

Mr Munro said the extra cost of reinstating a concrete footpath, instead of an asphalt footpath, meant "it can be concluded that the council is getting good value for money from the contractor".

The double-payment issue is the latest - and most serious - revelation from a five-month Herald investigation into the council's footpath contracts.

The revelations have led the Auditor-General to hold an inquiry into the management of the footpath contracts.

John Fillmore, whose company has been awarded contracts worth about $120 million since 2004, has welcomed an independent investigation.