Suggested changes to the act include explicitly excluding learning disability and autism from the act, as they aren't mental illnesses or disorders.
The Lib Dem move was flatly rejected by the home secretary, who ruled out any further changes to the act beyond proposals announced before the detention of Miranda.
The government's proposed changes to the act raised serious questions about national security, fuel security, jobs and skills, and the protection of the environment, he said.
Changes to the act are central to National's policies in its third term in government.The by-election was called after National's MP unexpectedly vacated the seat.
But Cash's office rebuffed his offer, saying: "I am more than happy to arrange for the Department of Employment to brief you on the proposed changes to the Act once the legislation is publicly available".
In the past week, a number of Coalition backbenchers have spoken out about the need to revive changes to the act, which were first promised by Tony Abbott ahead of the 2013 election.
