The comparison on the Australian bouldering site (it still works just has not been updated for ages) is as good as conversion as you will get. The grades of Perth boulder problems are about right when compared to this and other areas I have bouldered outdoors in (NZ, Sydney, Tassie and Victoria). There is so much conjecture of grades of routes, and boulder problems are worse as the moves are often more specific and the sequences short leading them to be easy for some types of climbers and hard for others.