When commentators look at the level of debt being serviced by Australian households they should take the funds deposited in mortgage offsets into account, according to the Reserve Bank.RBA head of financial stability Luci Ellis said the popularity of offset accounts made households more resilient to financial and economic shocks and it made the whole system safer.Offsets are deposit accounts linked to mortgages. Funds deposited into offset accounts are netted against the borrower's outstanding mortgage balance for the purpose of calculating interest on the loan.Offset account terms allow borrowers to withdraw the accumulated funds.Speaking at Macquarie University's Financial Risk Day last week, Ellis said: "The Australian offset account is the most tax-effective form of precautionary saving I have ever seen. "Not everyone with a mortgage is ahead of schedule but many are and those offset balances are growing rapidly."Last August the RBA reported that offset account balances stood at A$90 billion, representing more than six per cent of housing loans outstanding. They have been growing by around 30 per cent a year - a much higher rate then the growth in housing debt.Offset accounts are a form of mortgage prepayment but are not treated as such when the RBA measures changes in housing credit.Ellis said that in its recent work the RBA has been looking at the impact of offsets on growth in housing credit.Over the six months to June last year housing credit growth was seven per cent. Taking offsets into account, net housing credit growth was closer to six per cent.Ellis also had a few words of advice for commentators who look at growth in household debt as a proportion of GDP and conclude there is a household debt crisis."Households don't pay their debts out of GDP," she said."When you look at debt to household income there was an increase in the 1990s and early 2000s, and then a turning point around 2005. Since then it has been stable for a decade."