The rent information on the HDB website is declared by the owners themselves.

The authorities state on the website that the data has not been verified, while some industry insiders point out that it is possible for owners to under-report and unlikely to over-report, so the data is still of reference value.

As at 29 January, 223 four-bedroom flats were rented at $4,000 and above in December last year and January this year combined, compared with 114 in October and November last year. All municipalities on the island, including Jurong West, Sembawang and Woodlands, saw such high-priced rental transactions.

The biggest increase included Toa Payoh, which saw a sharp rise from four to 21 units. Choa Chu Kang, on the other hand, has gone from strength to strength, with seven high-rent units appearing in the past two months alone.

ERA's Head of Industrial Research and Consultancy, Mr Mak Chun Wing, Altitude Real Estate's Principal Executive, Mr Lim Kim Choy, and Arun Kumar, a housing agent who handles many Toa Payoh HDB flats, spoke to New Matinee to analyse the trend of HDB rents and the factors behind it.

Last year, there were about 31,000 units that reached the minimum occupancy limit, while this year, the number is expected to drop to 15,700. This means that there will be fewer new flats available for full occupancy in terms of market supply, so landlords will continue to be in the driver's seat.

Mr Lim said rents are expected to continue to rise as demand continues to be strong following the full opening of the border, the return of expatriate workers and students, the oversupply of university dormitories and the number of young couples renting out their homes while waiting for their pre-purchased HDB flats to be completed.

In the past two months, there has been a sharp increase in the number of four-bedroom HDB flats available for rent at over $4,000.

As to why Toa Payoh has seen a surge in high-rental units, Lam Kam-choi believes that the location is moderate and there are many schools, industrial areas and office buildings nearby, but whether there are other factors remains to be seen.

According to data from property website 99.co and the Singapore Real Estate Syndicate (SRX) report on rents for November last year, flat rents as a whole rose by 34 per cent compared to the same month the year before, and were higher than the 27.8 per cent for HDB flats.

In addition, Alan Kumar mentioned that those who take out bank loans to pay for their homes have to face higher interest rates and therefore demand higher rents to reduce their financial pressure.

He was looking for tenants for a four-bedroom flat in Toa Payoh and received a steady stream of enquiries and calls on the first day he put it online.

"Eleven groups were arranged to come and see the flat the next day, and then these potential tenants offered a higher rent than the $4,000 originally asked by the owner in order to get the flat."

Allen revealed that one group of six youths with student passes offered $4,500, but the owner eventually chose to rent the flat to a family of four from out of town for $4,300.