Fossilised or petrified mushroom

[Meowgi]

This appears to be a fossilised mushroom. Seems unlikely, I know, but precedents have been found recently. It could be mushroom coral...

 

I don't have a good camera phone, so I asked someone else to take the photos, I wasn't able to insist on following the forum rules for Id photos. Apologies.

 

Found while digging in the garden. It was already broken. Australia, NSW, Eastern Tablelands.

[Tidgy's Dad]

It would appear to be part of a corallite of a scleractinian coral. 

[Meowgi]

Thanks! I will look into the "corallite of a scleractinian coral ".

[Tidgy's Dad]

Something like this living example. (not the same species as yours) 

[Ludwigia]

I agree wholeheartedly with coral. Fossilized fungi are extremely rare and only preserved under extraordinary circumstances.