Sharesight does not appear to be using the RBA published exchange rates. Historical Data | RBA
The RBA exchange rates for the corresponding date as at the time of the transaction should be used where published for a currency pair, Sharesight appears to be using arbitrary exchange rates from Open Exchange Rates API and not these published RBA exchange rates. Exchange rates are published at 4pm AEST (NSW time) on business days. Conversion to AEST may be required to determine the applicable date on which the transaction falls. In the case of a transaction occurring on a weekend or public holiday, the previous exchange rate published should be used. However there does not appear to be an official determination with regard to which rate should be used on non business days.
More information: https://www.ato.gov.au/tax-rates-and-codes/foreign-exchange-rates-overview