The sight of a gigantic oil rig anchored off Perth beaches has beachgoers guessing about where it came from and where it’s going.

Ocean Monarch, the property of US firm Diamond Offshore Drilling, will have spent about 30 days sitting between City Beach and Rottnest Island as it has maintenance work before heading to Bass Strait.

The 105m wide deep-water platform has made the long journey from Singapore travelling at an average speed of 8km/h.

When fully operational the platform is home to more than 100 people and can drill to depths of almost 11,000m.