> when the Australians later sent highlanders to work on coastal plantations in the 1950s, they died at alarming rates

What is this referring to? Indentured Servitude? Slavery?

It says Malaria further in.

No, I am asking why were Australians sending the Highlanders anywhere? By what right? Why were they leaving?

Probably because living in PNG sucks and they would rather risk their lives at a chance of improving their condition. Same reason immigrants came from Germany to the US to live in mud dugout houses in the plains. Or immigrants go anywhere really.