Scenic Punggol Waterway Park is a favourite spot for young families, with many having picnics and flying kites there on weekends.

But after dark, another part of the park comes alive. In the forested area on a hill just a few hundred metres away from the park, a series of makeshift brothels pop up.

The Straits Times visited the area, located near the Safra Punggol club, in the evening on April 17 and found three clearings about 20m from the edge of the forested area overlooking the park, which is a haunt for foreign construction workers because of its proximity to their dormitories and worksites.

Each 5 sq m clearing, marked by joss sticks planted on the ground, had a blue tarpaulin sheet covering a mattress surrounded by condom wrappers, used tissues and empty water bottles. Used condoms, stained clothes and empty boxes of lubricant also littered the area.

Women dressed in black, believed to be prostitutes, could be seen climbing the slope into the clearings as night fell, even as park visitors continued cycling and jogging on the track below. Six men could be seen loitering in the area surrounding the clearings.

Workers in the area were aware of the makeshift brothels where 13 prostitutes are said to ply their trade almost every night, between 7pm and midnight, to about 20 workers of different nationalities. A worker, who wished to be known only as Sharif, said they charged $20 for each half-hour session.

Some said they believed locals also visited the prostitutes as they have seen people parking their cars nearby and walking up the slopes late at night.

Dr Goh Wei Leong, the director of HealthServe, which works with migrant workers, said that although jungle brothels are an ongoing issue, there are no figures on how many people visit such brothels.

The area had been cleared of tarpaulin sheets and mattresses when The Straits Times visited again last Friday morning, and a sign put up by the Singapore Police Force stated that eight people had been arrested for trespassing in the area, which is state land. Police confirmed the arrests but did not comment further.

In March 2014, 61 foreign nationals were arrested in the forested areas along Punggol for "illicit activities" such as vice and gambling.

However, the problem is believed to be a recurring one, as workers said the brothels have been raided several times in the past, only to be set up again shortly after.

Residents, many of whom have young children, said they were troubled by the presence of prostitution in the area.

Engineer Andy Herrera, 32, who is married with one child, said that although he did not mind workers congregating in the park, he was worried about the safety of his family.

Ms Nur Ali, who works in the mental health industry, said the brothel made her "nervous" as she sometimes had to work late.

"Knowing there's risque business going on not too far from here is unsettling," said the 31-year-old.