The road, which was closed to traffic on Wednesday, leads from the coast to the tourism destination of Patong Hill and has economic significance for the southern holiday island.
The landslide ate into a 500-metre section of the road, said Yutthana Phithak, director of Phuket District Highway Office. He said the road was built on a soil slope that had been undermined by heavy rain since last week.
Yutthana said officials have taken sampled soil layers at the site to determine their strength and will decide whether the road can be fixed and safely used or must be rebuilt entirely. The tests should take a few days, he added.
In the meantime, motorists heading to Patong are advised to use highways 4028, 4025 or 4030.
Phuket Governor Narong Woonciew said the province will try to make part of Highway 4029 usable as soon as possible, as it was the gateway to Patong Hill and an economic route used daily by both locals and tourists.
The province expects to see an influx of Thai and foreign tourists at local attractions over the year-end high season after Covid-19 restrictions were lifted.