Sydneysiders hold the dubious honour of being Australia's mortgage moguls, with remaining debts 30 per cent higher than the national average.

The average Sydney mortgage now weighs in at $365,873 across all mortgage-holding households, including those who bought decades ago, according to analysis by the Grattan Institute.

The analysis shows the national average for all outstanding principal mortgages, excluding investment loans, now stands at $280,912, prompting fresh warnings authorities will have to impose speed limits on mortgage growth within the next six months.

The average outstanding Sydney mortgage is now valued at more than $365,000. Credit:Sam Mooy

The figures reveal it is younger Australians who shoulder the highest mortgage burdens, having had less time in which to pay them down. Nationally, households where the reference person is aged 35 to 44 shoulder the biggest mortgage debt of any age group, at an average of $353,087 a household.