Leo Green conducts business as a Registered Tax Agent. The following occurred in relation to Leos clients affairs for the 2018/19 tax year:

• On 26 October, Patty Thomas engaged Leo as her Registered Tax Agent to prepare her 2018/19 income tax return. She has not used the services of a tax agent previously.
• On 19 November 2019, Carl Beck has engaged Leo as his Registered Tax Agent to prepare his 2018/19 income tax return. He has previously engaged Kennedy and Associates as his Registered Tax Agent.
• On 14 January 2020, Leo received an assessment for Mary Van for a tax return prepared by Leo for 2018/19. The assessment makes Leo aware that he has made an error in estimating her income tax payable. The return lodged was correct.
• On 26 May 2019, Leo received a letter requesting further information relating to deductions claimed in the 2017/18 tax return of Trish Bolt. Leo has prepared this return on behalf of Trish. A review of his file records shows that he has left our $8,000 of assessable income resulting in a tax shortfall of $3,200.

For each of the above points, provide Leo with advice as to what action he should take to minimise or prevent any penalties charged to his clients.