How did the changes that augustus made to the roman tax system reduce government corruption

He did that by making tax collectors permanent Government officials and paid them Regular Wages.

Before Augustus' new policy, tax collecting was being done by Publicanis ( in modern world, they would be some sort of public contractor). These Publicanis often give the Government a smaller tax payment than they actually collect. By cutting them off the system and paying the new collectors Regular wage, Augustus manage to prevent this occurrence from happening again