Nearly 4 million Australians relying on voluntary super contributions were falling behind in what they needed for a comfortable retirement, according to an AMP report.

The AMP superannuation adequacy index for 2008 revealed 37 per cent of working Australians were falling short of adequate retirement targets by an average of $3,821 annually.

This is a 6.8 per cent increase since AMP's report in December 2007.

The report attributed these figures to the reductions in member balances and voluntary contribution rates resulting from current market conditions.

Super balances fell by 3.1 per cent per member, with older Australians the hardest hit.

Balances for those aged over 55 years fell by 8.9 per cent.

The report revealed that superannuation contributions decreased to 12.6 per cent for the six months to June 2008, down from 17.2 per cent in the June 2007 data.

Despite the declines, AMP financial services managing director Craig Meller said that super adequacy was expected to pick up over the longer term as conditions improve.

"While market falls for the first half of 2008 have impacted super balances - and will continue to do so in the short-term, increased investor confidence when markets recover is likely to reverse much of this decline," Meller said.

"So assuming a return to long-run rates of return over the next few years, and that voluntary superannuation contributions do not continue to decline, it is likely that 2007-2008 super returns will do little to reduce retirement income adequacy for Australian workers over the long-term."

The fourth AMP super adequacy index examined data for the December 2007 to June 2008 period.