The median house price jumped from R580000 in April to R600000 last month, according to Standard Bank's residential property gauge.
Average growth in house prices for the year to April was 11percent.
"The relatively strong house price growth recorded in May is in our view artificially inflated by pre-emptive actions of some property buyers, real estate agents and mortgage originators prior to the implementation of the National Credit Act rather than a reflection of the underlying economic fundamentals driving the property market," Standard Bank said.
Last month some mortgage originators and real estate agents made a concerted effort to tie up as many property purchases as possible before the implementation of the act on Friday. This was because of the perceived tightening in banks' lending that they feared might might follow.
"The strong house price growth recorded during May was artificially inflated by the pre-act activity of some property buyers, real estate agents and mortgage originators. Therefore we haven't altered our view that house price growth should remain subdued in the near term, especially if monetary policy is tightened. The medium-term outlook, in contrast, is still buoyed by a constructive macroeconomic setting," the report said.
Standard Bank has five house price indices segmented from low to high-value homes. Price growth in all segments has tailed off from a peak last year.