They are pretty birds with a sweet whistle but increasing numbers of eastern rosellas could be a problem, environmental scientist Peter Frost says.
In the early 2000s an atlas of New Zealand birds recorded the Australian parrots on State Highway 43, the Forgotten World Highway through Whangamomona. By 2009 they had moved south and people were noticing them in Ranana and Jerusalem on the Whanganui River.
At first members of the Birding Whanganui group used to report seeing them.
"Now people don't even bother to report them - they're all over the place," Mr Frost said.
But there could be downsides to having them there.
They nest in cavities, especially holes in trees, which are also favoured sites for endangered native hihi (stitchbirds) and kingfishers. Luckily, the rosellas are too big to fit into the nestboxes most hihi are using.
Perhaps the worst possible problem is disease. Parrots host a range of diseases and the presence of rosellas could be a limiting factor if native kakariki parrots were to be reintroduced to Bushy Park.
Eastern rosellas are native to the entire eastern coast of Australia, from Cape York to Tasmania.
"They're marvellous things in Australia, but they seem completely out of place somewhere like Bushy Park," Mr Frost said.