The 48-year-old officer and 54-year-old former policeman were arrested along with a 37-year-old member of the PSNI’s staff and a 62-year-old civilian.
They were detained in counties Down Antrim and Tyrone. Two men, aged 34 and 54, were also arrested in England as part of the large scale investigation.
The six suspects have been taken to Antrim police station. Police also searched residential and business properties and have seized several items.
A PSNI spokesman said: “The police service expects its officers and staff to behave professionally, ethically and with the utmost integrity at all times.
“Any conduct which brings or is likely to bring discredit on the service will be investigated to establish whether or not a breach of the code of ethics has occurred.
“This may, as a result, lead to a criminal investigation.”
A Policing Board spokeswoman added: “The board has been informed of an ongoing PSNI investigation into the awarding of a number of PSNI vehicle contracts.
“This is a matter of very serious concern and the board will be seeking an urgent update from the Chief Constable on the progress of the investigation.”