Unidentified

The lens cap is 50mm across

I suspect this is Cortinarius australiensis due to the size. The other posible species is C. sublargus that is more common after fires, though tends to be slightly smaller. This later species is distinguished by a short stem (stipe) above ground and a deeply rooting section underground (up to 20 cm!). Is it possible to check this? Impressive fungus and find - how exciting, lucky you!

Not sure to check the rooting section Pam without destroying the plant. Or will the spores continue to be viable after the plant is removed from the ground?

If you just have a scratch down one side you will soon find out if the stem is conical and continues down or if it is bulbous as is the case with C. australiensis. Regardless, the gills have coloured up, so the spores are mature therefore if you leave it in situ the spores will continue to disperse. This is how we get spore prints, by placing the cap, gills side down, on paper and the fungus continues to drop its spores even though it's been removed from its natural habitat.