It was part of the board's disciplinary fee until 2010 when Parliament's regulations review committee discovered the money being collected from licensed members was being spent on prosecuting non- members.
That, said the committee, was illegal so in order to get around the problem, the board started charging a separate "Offences Fee".
However the regulations review committee is about to issue a ruling on a complaint that the offences fee is also illegal for the same reason.
Mr Williamson's legislation would retrospectively allow the board to collect fees for the purpose of prosecuting unregistered persons.
Without it, "the board could be left unfunded to carry out its statutory prosecution functions", he said.
"The board would be placed under significant financial pressure and would be unable to carry out its important statutory functions that directly affect the safety and the well-being of New Zealanders."
Green MP Holly Walker said there was a risk that if the offences fee was disallowed, the board would be forced to refund the money it had collected, leaving it in a "financially unviable position".
Labour's Raymond Huo attacked Mr Williamson's use of retrospective legislation, noting allegations that it was being used to protect the board from legal action and prevent the industry from recovering money illegally taken from it.
"This bill is nothing more than a cynical exercise by the minister ... to cover his mistakes of the past."
Labour will support the bill to select committee to give interested parties the chance to have their say.