They lifted heavy grates off the drains, using flash lights and headlamps to search inside.

Update 8pm: Police are concentrating their search efforts on a drain at the edge of the park, where the majority of the runoff has pooled.

They are also lifting the covers off drains across the road from the park where rushing water can be heard.

One woman has taken to social media to say the park often becomes a popular sliding ground during heavy rainfall. 

“We saw them, about 20 kids with their boogie boards, it's something kids do there all the time. So devastating,” she wrote.

A resident, who didn’t wish to be named, told the Mercury: “they bring their body boards and race down the hill”.

7pm: There are fears for an 11-year-old boy presumed swept away in flood waters at Unanderra. 

Police, on scene, confirmed the boy was last seen riding a boogie board at flooded Riley Park off Central Avenue.

The boy went missing about 4.30pm.

A search is underway, involving a large number of SES volunteers, police and Ambulance Special Operations Team paramedics (highly trained clinicians who are also trained in swift water rescue). 

As at 6.30pm the search has focused on stormwater drains in the vicinity of the park. 

Distraught onlookers are watching the search unfold. 

The park becomes a popular sliding ground during heavy rainfall. 

Children have continued riding boards down the green hill Thursday evening, even as the search for the missing boy goes on.  

Authorities have consulted the boy’s family multiple times and are convinced he is missing. 

MORE TO COME