Wichit Police explained that the closure is necessary so the new pump can be installed to help prevent further, repeated, flooding of the tunnel during periods of heavy rainfall.
Motorists have been advised to avoid the area, or prepare to experience delays as traffic is diverted around the tunnel.
Traffic Police are redirecting southbound vehicles on the bypass road onto Phra Phuket Kaew Rd through Kathu.
Wichit Police did not estimate when the southbound lane through the underpass will reopen fully to traffic.