If you forget to declare any income from previous years, you are at risk of the ATO auditing you to correct your tax return. You can also voluntarily amend previous year tax returns though you should consult with a professional before doing this" says cryptocurrency tax expert and advisor, Harrison Dell.
"If you forget to declare some losses in previous years, it may increase your risk of being audited by the ATO, but there are usually no penalties for not reporting a loss. However, if you forget to report a tax loss in a previous year it doesn't stop you for using it in a later year. This applies to both capital and income losses. It is much easier to use a prior year tax loss if you have reported it correctly, as it will ensure the your tax returns are consistent.