It said higher borrowing costs for buyers had led to a slowdown in activity in the housing market.
Mortgage approvals are also about 20% below pre-Covid levels.
Since December 2021, the Bank of England has lifted interest rates 14 times in row in a bid to clamp down on rising consumer prices in the UK. The bank's base rate now stands at 5.25%.
That, in turn, has led to lenders raising their mortgage rates, putting increased pressure on homebuyers.
Nationwide's chief economist Robert Gardner said the rise in the cost of borrowing meant the fall in average house prices was "not surprising".
He told the BBC's Today programme that affordability for house was "much more stretched than it was before" pointing to a typical rate now being towards 6% compared to 1.5% in late 2021.
"Clearly that has had a big impact on the market as a whole," he added. "I think it is going to take time for things to pick up at all."
According to financial information service Moneyfacts, the average two-year fixed mortgage rate on Friday was 6.7%, while the average five-year fix was 6.19%.
Average house prices in the UK peaked at £273,751 in August 2022 but fell to £259,153 last month.
While the drop is the biggest since 2009, property prices are still much higher than they were in August 2021 when the average was £248,857.
Nationwide is one of the country's biggest mortgage lenders. But the building society's figures only take into account buyers with mortgages and do not include those who purchase homes with cash or buy-to-let deals.
According to property website Zoopla, people with mortgages currently make up 60% of all house sales, compared with 31.8% cash-buyers and buy-to-let making up the remaining 8.2%
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