The Dangerous goods legislation points to ICAO Doc 9284 for the definition of dangerous goods.
The only snag with that is that document is only available to commercial operators and not to Joe Public.
There are many things that the airlines prohibit, but are regularly carried on private flights like fire extinguishers and gas bottles in life-jackets and rafts.
I can't recall that the act has ever been invoked for a private flight.
In the UK petrol is a forbidden item on private flights according to the UK CAA Advice on the carriage of Dangerous Goods (Private Flights).
Just look here under flammable liquids.
Dangerous Goods | Aircraft | Operations and Safety