The standards commissioner received nine complaints in the past year, down from 14 in 2015-16 and 53 in 2014-15.
But Douglas Bain said the drop was "due in a very large part to lack of public confidence in the complaints process and in the Assembly".
"No right-minded person could seriously believe" it was because politicians were "behaving better", he added.
Mr Bain, the first Northern Ireland Assembly Commissioner for Standards, told the BBC there were a number of a problems with the complaints process at Stormont.
He said the system was not transparent and he was forbidden by law from revealing details of the complaints, even if admissible.
The dominance of party politics was also an issue at Stormont when it came to complaints, he said.
"In every other jurisdiction in the UK which have equivalent processes, they leave their party politics at the door of the committee room," he said.
"That simply doesn't happen here".
The commissioner's main function is to investigate complaints that Members of the Legislative Assembly (MLAs) have broken the code of conduct and to report his findings to the committee on standards and privileges.
Mr Bain said the controversial petition of concern - a Stormont mechanism intended to protect one community from legislation that would favour another but which has been used as an effective veto - had "been used in relation to conduct matters".
"Even when they (the committee) agree a finding by me that the MLA has breached the provisions of the code of the conduct, it is up to the Assembly to decide on any sanctions," said Mr Bain.
"On two occasions, a petition of concern has been put down, which has prevented the imposition of any sanction on someone who has been found by an independent investigation and the committee to have breached the provisions of the code.
"The public simply cannot have confidence in a system that operates like that."
He said little or no action had been taken on last year's recommendations and he called for urgent action on these main issues:
The latest figures are contained in the standards watchdog's annual report - Mr Bain's fifth and final review of his work. The Office of Commissioner was established in 2012 and his five-year term in office ends in September.
When asked if he would recommend his job to anyone, he said it was "interesting" and "challenging".
"But it is frustrating when the public are prevented from getting the system they are entitled to by the way some parties react to the issues," he added.
The number of complaints about the conduct of Assembly members has fallen to its lowest level in five years.