August saw prices increase by 2.1% month-on-month, the second-largest gain in 15 years.
But Gardner warned that despite the current positive market conditions the longer-term outlook is still clouded.
He said: “Underlying demand is likely to remain solid in the near term.
"Consumer confidence has rebounded in recent months while borrowing costs remain low.
"This, combined with the lack of supply on the market, suggests continued support for house prices.
"But, as we look towards the end of the year, the outlook is harder to foresee.
"Activity will almost inevitably soften for a period after the stamp duty holiday expires at the end of September, given the incentive for people to bring forward their purchases to avoid the additional tax.
“Moreover, underlying demand is likely to soften around the turn of the year if unemployment rises, as most analysts expect, when government support schemes wind down.
"But even this is far from assured. The labour market has remained remarkably resilient to date and, even if it does weaken, there is scope for shifts in housing preferences as a result of the pandemic to continue to support activity for some time yet."