The main road out of Wollongong could remain closed to most traffic for some time, due to the risk that a private dam could release 450,000 litres of water over the M1. Transport for NSW said the decision to close off Mount Ousley to all northbound traffic for most of the day on Tuesday was not taken likely. "The closure was put in place to protect life and property during this severe weather event," a spokesperson said, noting the dam had the "potential release of an estimated 450,000 litres of water over the M1 near Mount Pleasant". READ MORE: "This situation presents a high safety risk to road users and all agencies are taking a precautionary approach." Overnight on Monday, emergency services were called to inspect the integrity of a dam at a property on Parrish Avenue, Mount Pleasant. During the day, NSW Police, SES, Rural Fire Service and traffic engineers attended the property, near the Wollongong Rhododendron Gardens, to help secure the dam. In a video posted to social media, the Wollongong City Unit of the SES said the property had a dam at capacity which was threatening to flood over Mount Ousley Road. Emergency service workers can be seen using pumps and sandbags to redirect the water elsewhere on the property, which backs onto Mount Ousley Road. "We've been pumping and diverting water away from the road so that it can reopen to traffic," a spokesman said. Trucks were queued for hours on Tuesday morning south of the road closure, until one lane of the road was reopened to allow heavy vehicle traffic through. All other cars have been told to detour via Memorial Drive and Bulli Pass. Lawrence Hargrave Drive is now also reopened after an earlier landslide. It is unclear when the road will be able to reopen, due to the continuing high level of rainfall and water volumes still entering the dam. Crews are on standby to reopen the motorway immediately when it is deemed safe.