The important thing to understand here is that while a “90% rule” does not exist, “considering” the diversity of your investments is a requirement and an important part of running your SMSF responsibly.
Within your SMSF you will have an “investment strategy.” The strategy needs to reflect the purpose and circumstances of your fund and one of the points of consideration here refers to diversification.
Other items that you are required to consider in implementing and reviewing your investment strategy include taking into account maximising returns, the needs of the members in retirement, how long they have until retirement etc.
Below are some links to the ATO that outline the requirements being addressed here.
Running a self‑managed super fund (page 15)
As long as these items have been “considered” then you have satisfied the requirements.
While we cannot offer financial advice, the old mantra of “not putting all your eggs in one basket” is a wise one.