I don't know about Sri Lanka celebrants or registry offices but keep in mind your partner must enter Australia once after the 300 is granted before you marry.
This may be a good starting point:Hi Nick,
Thank you for response.
We understand that we can conduct a small civic wedding in Australia and opt to have the traditional ceremony in Sri Lanka.
But are you aware of a method wherein we can conduct the registration/ wedding ceremony in one go at one place outside of Australia (Sri Lanka)? If so, who do you reckon we must approach?