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The ageing of trade receivables at the reporting date is detailed below: 

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The consolidated entity’s policy requires customers to pay in accordance within agreed payment terms. Depending on the customer segment, settlement terms are generally less than 30 days from date of invoice. An allowance for doubtful debts is recognised when there is objective evidence that a trade receivable is impaired. Financial difficulties of the debtor, default payments or debts overdue are considered objective evidence of impairment. An impairment loss for doubtful debts of $66.9 million (2011: $62.6 million) has been recognised in the year and has been included in other expenses in Note 7. 

At the end of the reporting period, trade receivables with a carrying amount of $166.8 million (2011: $133.3 million) were past due but not considered impaired. These trade receivables relate to customers for whom there has not been a significant change in credit quality and the amounts are considered recoverable. 

Other balances within trade and other receivables are neither impaired nor past due. It is expected that these other balances will be received when due. 

## Unbilled revenue 

Unbilled gas and electricity revenue is not collectible until such time as customers’ meters are read and bills rendered. 

## Amounts owing by associates and jointly controlled entities 

For terms and conditions relating to amounts owing by associates and jointly controlled entities, refer to Note 50. 

## Fair value and credit risk 

Due to the short-term nature of these receivables, their carrying amount is assumed to approximate their fair value. The maximum exposure to credit risk at the reporting date is the carrying amount of each class of receivables. Collateral is not held as security. 

