The uncollected council tax arrears is described as "staggering"

Welsh Lib Democrat leader Mike German highlighted Monmouthshire council where �89 is owed per household, compared to �55 arrears owed on average in Wales.

However, the council said their tax collection rates were improving after installing a new computer system.

Latest assembly government figures show across Wales, �71.3m arrears are owed.

Mr German, who is AM for South Wales East, said that local councils in Gwent were "struggling to reduce a mountain of debt."

He added: "And our local experience is being repeated across Wales and in councils across the UK.

"Every pound uncollected means more money has to be paid by those that do pay."

According to figures released by the assembly government's National Statistics department, Monmouthshire was the local authority in Wales with the most arrears outstanding per household.

At the end of March 2007, �89 was owed per dwelling against the Welsh average of �55.

Blaenau Gwent with the second highest at �80 per dwelling while Pembrokeshire council performed best with under �20 per dwelling.

COUNCIL TAX ARREARS Anglesey �2m Blaenau Gwent �2.56m Bridgend �4.2m Caerphilly �3.3m Cardiff �9.6m Carmarthenshire �4.68m Ceredigion �2.48m Conwy �1.74m Denbighshire �1.3m Flintshire �1.59m Gwynedd �1.79m Merthyr �1.69m Monmouthshire �3.36m Neath Port Talbot �3.25m Newport �2.3m Pembrokeshire �0.9m Powys �4.34m RCT �5.67m Swansea �6.65m Torfaen �1.82m Vale of Glamorgan �1.86m Wrexham �3.9m Total arrears outstanding, source: Statistics for Wales

However, a spokesperson for Monmouthshire council said a new computer system would provide a "more efficient council tax system".

"We also hope to install a new cash receipting system during this financial year, which will give council tax payers more payment options and therefore hopefully increase the collection figures," the spokesperson added.

The council said it wanted to encourage people to pay their council tax by direct debit as it lessened "the financial impact on residents, reduces overall collection costs, creates a certainty of income for the council and avoids unnecessary recovery costs."

But the council said it did not look to collect uncollected council tax directly from other council tax payers, if there was a shortfall in income.

It said it would be met from the council's overall finances, which includes council tax.

Overall, the total amount outstanding in Wales at the end of March was �71.3m, a rise of 5% on the previous year.

Welsh local authorities collected 96.6% of council tax owed in 2006-07, the same as the previous year, but 10 of the 22 councils managed to increase their collection rate.

Powys recorded the highest - collecting 98.8%.

But Mr German, whose party have backed a local income tax, said he thought that the council tax system was flawed.

"It's expensive and complex to collect and by the looks of the latest figures too many people are not paying their share," he said.