According to data from the Housing Development Board and the company, only about six four-bedroom flats resold for more than $1 million in the same period last year, Bona's head of property research and content Wong Sau Ying told MediaCorp's English news website TODAY.

She believes that the government's requirement in September last year that private home owners must wait 15 months after selling their homes before they can buy resale HDB flats may have pushed up demand for four-bedroom HDB flats, prompting more units to change hands for a million recently.

This is because owners aged 55 and above who move from private homes to four-bedroom or smaller resale HDB flats are exempted from the waiting period.

According to Sun Yanqing, senior vice president of research and analysis at OrangeEasy, more people are turning to HDB flats near the city centre and are willing to pay more than $1 million as private property prices climb.

The real estate expert said that all the million-dollar four-bedroom resale HDB flats are located in established towns, mostly in the Redhill and Central areas.

Experts believe that it will become increasingly common for resale prices of four-bedroom HDB flats to exceed one million, and senior real estate industry analyst John Mak estimates that this will remain the case for the next 10 years.

He believes that this is partly due to the absence of five-bedroom flats in the recently launched pre-built HDB projects in popular areas and prime locations in established towns.