ANZ National's asset quality has worsened the most this year compared with its peers - Westpac, ASB and Bank of New Zealand, due to rising impairments in its corporate exposures.The bank's impaired assets more than doubled in the nine month period to June to NZ$1.80 billion, compared with NZ$871 million in the same period last year.Nearly 68.2 per cent of the impaired assets are corporate loans, up from 60 per cent last year. ANZ transferred a huge NZ$919 million of assets to the impaired category during this period. The increase reflects higher numbers of defaults within the rural and commercial portfolios of the bank.In percentage terms, however, BNZ has been the worst performer where corporate exposures are concerned, with 78 per cent of its total impaired assets belonging to this category, up from just 38 per cent last year.  The increase is primarily in commercial property, business lending and in the dairy farming sectors.Westpac has performed relatively well with its corporate exposures at 50 per cent of total impaired assets, and the ratio has dropped from 58.5 per cent last year. The bank, however, has fared badly as far as retail mortgages are concerned, with 50 per cent of total impaired assets comprising mortgages. This has risen from 41.5 per cent last year. Westpac blames it on the "severe correction in asset prices."ASB has seen a worsening in its residential mortgage portfolio with impaired assets rising to NZ$186 million in the year ended June, up 116 per cent from last year. The bank's corporate exposures also rose to NZ$292 million, up 13 per cent. Impaired assets belonging to the corporate category comprise 59.3 per cent of total impaired assets.Surprisingly, none of Westpac's other retail loans are impaired, whereas for the other three they are around 3-4.5 per cent of total impaired assets.Westpac is the only one among the four to see a drop in its impaired assets, falling to NZ$760 million from NZ$832 million last year. For BNZ such assets rose to NZ$746 million, up 69 per cent from last year; in the case of ASB impaired assets rose to NZ$492 million in the 12-month period, up 40 per cent from last June; and for ANZ, as mentioned above, the rise has been 206 per cent.