The Walkinshaw Andretti United driver finished fourth in today's first of three races at the Hidden Valley circuit.
However following a post-race investigation the result won't stand, his Walkinshaw Andretti United crew found guilty of illegally using a battery-operated air blower on the car on the grid.
"Rule D8.1.8 prohibits the use of any device to artificially vary the temperature of any part of the car when it is on the grid with the exception of driver cooling," read the stewards report.
"The [Head of Motorsport] observed a battery-operated blower inserted in a cavity in the front bumper of car 25 when it was on the grid for Race 16. The device was activated. It was blowing air onto the radiator of Car 25.
"The authorised representative did not dispute that the device had the effect of lowering the temperature of the engine cooling system on car 25 from the temperature build-up achieved on the reconnaissance lap and that, as a result, car 25 had an advantage over other cars at the start of Race 16.
"The authorised representative acknowledged that Rule D8.1.8 was breached.
"The authorised representative explained that the device had been used by a member of the team without consulting him and without his knowledge. He explained that the driver had reported an issue with the car on the reconnaissance lap and that he had been occupied attempting to identify and resolve that issue.
"While the stewards accept that the breach was not deliberate, it nonetheless represents a technical infringement for which the universal penalty is disqualification.
"Given that in this case the breach resulted in a potential sporting advantage for car 25 at the start of Race 16, the stewards are satisfied that no other penalty would be appropriate in this case."
This is Mostert's second disqualification in as many days after a tyre breach saw him kicked out of yesterday's second practice session.
As a result Andre Heimgartner moves up to fourth in the final classification of today's race behind Anton De Pasquale, Will Davison and Shane van Gisbergen.
