A wiring problem appears to have been the cause of two bus fires in as many months.

The NSW Office of Transport Safety Investigations has completed it investigation of a bus owned and operated by Linq Buslines.

The coach caught fire in September last year, luckily it was carrying only the driver and a single passenger and neither were hurt.

A motorist noticed smoke and flames billowing from the rear of the bus and was able to alert the driver to pull over.

Investigations of the event revealed electrical short circuiting of the alternator wiring caused the fire.

The company then loaned the same bus to Port Stephens Coaches in November, only for it to catch fire again.

The coach burst into flames while 24 people were aboard. All escaped, but the vehicle was totally destroyed.

Investigators say the second blaze was likely due to the same electrical short circuiting, but the exact cause will not be known due to the damage.