Johnston has pleaded not guilty to disclosing official secrets to Mr Lennon and current Police Minister Jim Cox in April last year.
Johnston is charged with illegally telling them the details of a police investigation involving cabinet ministers who were potential suspects in a corruption probe to do with two senior judicial appointments.
Mr Lennon told the Hobart Magistrates Court on Wednesday that it was highly probable he knew about the "secret" before Johnston told him on April 11, 2008.
His comments change his earlier evidence.
Mr Lennon had said in a statutory declaration that he first learned of an investigation involving cabinet ministers and two judicial appointments when Johnston told him on that day last April.
On Wednesday, under questioning by defence lawyer Terry Forrest QC, Mr Lennon said it was "highly probable" he knew about a police investigation before April 9, 2008.
Mr Lennon also said Johnston was duty bound in his role of secretary to the Department of Police and Emergency Services to provide him with a full and detailed brief of the investigation at that meeting so as to allow him to answer an opposition question in parliament.
Mr Forrest later described Mr Lennon's inconsistency as being "simply a witness's memory being better with time and thought".
Tasmania's Solicitor General Leigh Sealy also gave evidence to the hearing on Wednesday.
He also said Johnston was legally bound in his role as a department secretary under the Westminster system to provide timely, full and accurate the advice to Mr Lennon and Mr Cox.
"No servant of the crown has the authority to act unlawfully," he said.
The case continues.
aap 25 Mar 2009