She said the veteran faced an absurd and unjust situation.
"The Chatham Islands have officially been part of New Zealand since 1842. If the Government has decided that's no longer the case then they should have let on," Mrs King said.
"On the other hand, if this is just some officious bureaucrat trying to penny-pinch, then they owe Mr Thorpe an apology at the very least.
"Not only that, but the ocean between the mainland and the Chathams is not international water - it's part of New Zealand's exclusive economic zone and contains some of our best fishing grounds."
Mrs King said Mr Thorpe had noted there were probably only a handful of veterans who would ever want to access the concession to travel to the islands.
The Chatham Islands were part of Mrs King's electorate.
This afternoon, Veterans' Affairs Minister Craig Foss said the policy would possibly be changed.
But Mr Thorpe would have to wait a few months.
"The minister has been advised by Veterans Affairs that this has been longstanding policy," Mr Foss said in a statement to NZME News Service.
"However Veterans Affairs are reviewing this policy as part of their work to implement the Veterans' Independence Programme, which will be established from 7 December 2015 and will include the travel concession as part of a coordinated package of support."