"Hudson pear was first detected in Australia in the Lightning Ridge area during the late 1960s. It is believed to have spread from a cactus nursery at Grawin. Some reports state that this process was aided by opal miners who deliberately used the plants to protect their diggings from nocturnal prowlers and thieves but these are unable to be verified."

There ya go. Some dopey nurseryman brought them in for people to put in their gardens.

Unfortunately, the cacti are NOT there as a result of nature, but rather people deliberately putting them there. Australia has a lot of problems with exotic (to them) flora and fauna being brought to the island continent and then running wild on their rather fragile native populations of plants and critters. Rabbits & pigs transplanted from Europe years ago are a big problem for one.

I don't know the specifics of the cacti, but they may have been brought in for the horticultural trade -- that has proven to a big problem for certain invasive plants (like Kudzu & Burning Bush) in the U.S. for example.

Get out the Round Up. Spray 'em all.