Proposed "vote-winning" law changes relating to criminal justice reform of rape cases could result in more innocent people being sent to prison, the Criminal Bar Association warns.
National and Labour say the current system is not providing justice for victims of sexual violation, and reform would make the system more victim-focused.
National wants to explore whether a judge or jury should be able to see a defendant's refusal to give evidence in a negative light. Legal experts have called this an attack on the right to silence.
Labour wants the Law Commission to look at an inquisitorial system where the judge interviews the victim, and proof of consent has to be proven by the accused, if other aspects of the case are proven. This has been criticised as eroding the presumption of innocence.
Criminal Bar Association president Tony Bouchier said both ideas were "ping-pong policies".
"We are talking about the basic presumptions in the common law - the presumption of innocence, and the right to silence. These are absolute, almost constitutional, and to have political parties playing ping-pong with such important presumptions is just not a good thing. We are not open to either idea at all."
He said defendants should be able to choose not to give evidence, and should not have to prove elements of the case such as consent.
Both policies would swing the system in favour of the prosecution and might lead to more innocent people being sent to prison, Mr Bouchier said.