Metlink confirmed the bus had no passengers and was not in service.
Staff onboard the bus were safe, a Metlink spokeswoman said.
A photo posted by the agency shows a thick cloud of white smoke coming from behind an overbridge.
Police are controlling traffic.
A local posted on a community Facebook page to warn of the disruption.
“We just drove past a small Metlink bus fully ablaze between Porirua and Tawa (Southbound) on SH1 - avoid the area if you can!” the post said.
It comes after an Auckland Transport bus caught fire on the city’s North Shore this morning.
The bus driver and another person were hurriedly trying to fight the blaze with fire extinguishers while they waited for firefighters.
Auckland Transport said an out-of-service bus with no passengers on board caught fire outside the Birkenhead depot at 8.48am.
“Staff noticed smoke coming from the engine of the diesel-powered bus, and a crew from Fire and Emergency acted quickly to extinguish it.”
There were no injuries, and the cause of the fire would be investigated, AT said.