The Labour Party has released papers showing the lawyer acting for ACT Party leader John Banks argued against police releasing any files from their donations inquiry.
Police decided not to lay charges following their investigation into Mr Banks' electoral returns for his 2010 Auckland mayoral campaign and donations that had been declared anonymous.
When police released files from the inquiry, Mr Banks' statement to police was blanked out.
Labour deputy leader Grant Robertson said on Tuesday a letter from Mr Banks' lawyer to police released under the Official Information Act makes it clear that he did not want his witness statement in the public arena.
"Well, Mr Banks keeps telling New Zealanders that he's got nothing to fear and nothing to hide. Yet he's tried to cover up his own witness statement that he gave to the police."
Mr Robertson is now requesting the Ombudsman release Mr Banks's statement.
Mr Banks would not comment on Tuesday.