It's probably the dumbest conspiracy theory around. Most, perhaps all, of the worst bushfires are in places that are utterly unsuitable for high speed rail because they are mountainous, and well away from the proposed line. Why do they think that burning the trees in a national park or state forest is going to make it easier to put a high speed railway line through it anyway?
The biggest fires include the one in the Snowy (well away from proposed railways), the Carrai Creek etc complex inland of Coffs and Kempsey (a vast area, almost all well away from the proposed lines and in mountainous country), the South Coast fires (about 150 km from any line), etc. The Green Wattle Creek fire seems to have been worst around the old Picton Loop Line, which was discarded in favour of the better current railway alignment many years ago. And if you WANT a high speed rail line then you'd promote the idea of living in the country so you get more customers, not try to drive your potential customers to move back to the city because of fires.
It's also an insult to the RFS volunteers that they wouldn't notice or say anything - they are on the ground and would notice lasers flicking around, as would others in the area.
There's a conspiracy by conspiracy theorists - they are driving to drive sane people mad with their idiocy.

Then again, maybe it's an anti-conspiracy conspiracy. Y'see, if everyone use high speed rail then there'd be fewer airliners to be used to drop chemtrails on everyone, which would stop the guys who are running the Great Chemtrail Conspiracy.