United Future leader Peter Dunne has compared Australia's detention centres to concentration camps and says the Government's ministers are "villains" because of their poor treatment of detainees.
Mr Dunne made the comments in a regular blog "Dunne Speaks", and has repeated them in Australian media.
He said that in the uproar in Parliament this week over Mr Key's comments about rapists, the issue of detainees in Australia's offshore camps had been pushed into the background.
Mr Dunne said the some of them had committed unspeakable crimes, but they still retained the same basic human rights.
However, the "modern concentration camp" approach taken by Australia meant the detainees were now worse off than they were in prison.
Australia's offshore detention camps were "no different" to Guantanamo Bay because detainees were denied basic rights, Mr Dunne said.
"I suspect most New Zealanders are far from comfortable with the notion of holding such people captive on offshore islands, and would not let a New Zealand government even consider doing so."
While New Zealand could not interfere with Australia's sovereignty, it should be speaking out "as loudly and frequently as we can", especially because Australia described New Zealand as "family".
"After all, most families are blunt with each other and speak out about what they do not like. We should be as well."
Mr Dunne said that instead of fighting each other in Parliament, political parties should be targeting "the real villains of the piece" - Immigration Minister Peter Dutton and others who had promoted "savage and inhumane" policies.
The United Future leader also said Australia had previously shown a "frontier" approach to justice in its treatment of indigenous people and asylum seekers.