Do your bassinet standards for a mattress also apply to a cot if we are using a cot at the bassinet height? Your guidance specifies firm, flat, not larger than 75cm. If a cot passes the Australian safety standards, do we assume the cat mattress is also considered safe as part of the cot?

The recommended guidelines are the same for a bassinet - firm, clean, well-fitting & flat (& no greater than 75mm thick)
All safety standards are developed & monitored by Product Safety Australia (ACCC)
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/bassinettes

A cot mattress is not included in the safety standards for a cot, although it would be unusual for a mattress sold to not meet the voluntary standard for mattress firmness.
Check on the information with the mattress to see if it meets the voluntary standard for firmness.
The Mandatory Standards for all cots sold in Australia will also be available with the cot when sold.
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/household-cots