The head of the Electoral Commission says it is almost impossible to interpret parts of the new Electoral Finance Bill.
Dr Helena Catt, the commission's chief executive, said there were parts of the new electoral finance bill - which was reported back to Parliament with major amendments on Monday - which made it hard for the commission to do its job of advising on the law.
The problematic clause 80 says parties' election expenses do not include anything done by an MP if it was "in his or her capacity as a member of Parliament".
Dr Catt said there was no definition of the clause, which was open to differing interpretations.
The clause made it through the select committee unamended, despite both the commission voicing concerns in advice to the select committee, and the Chief Electoral Officer saying administering the provision would be "very difficult" without further clarification.
Dr Catt said the clause had never been defined or clarified - which was essential for the commission to advise the public and politicians and monitor election activities to refer suspected breaches to the police.
The Law Society also said the complexity of the law was one of its major flaws.
President John Marshall said the general public would struggle to understand it. Mr Marshall said the law society was concerned with the speed in which the law was being rushed through. "In our experience, hasty legislation is usually ill-considered and contains defects."
He said the Law Society still believed the bill should be scrapped and begun anew, or, at the very least, returned to the select committee for public submissions on the revisions.
"Our electoral law exists for the benefit of the people of New Zealand and they have the right to be heard on this substantially amended bill."
Dr Catt said the Commission also faced a major task making sure all parties, candidates and third parties could deal with the new donations regime, which had become far more complex.
However, the number one problem for interpretation was clause 80.
"If the definition of this is unclear, then it could see the MP concerned having one interpretation, the person making a complaint having a different interpretation, [the Electoral Commission] might have a different interpretation from both of them, and then the police may have a fourth interpretation."
Yesterday, National Party deputy leader Bill English said it was a "cop-out" by the select committee not to address the commission's concerns. "They should have either defined it or left it out. There is a lot of new law in this, it's all uncertain and the process of sorting it out will be messy and expensive."