It’s a tricky time to be planning a wedding, with rules changing frequently.
Here is the latest, on a state by state basis, with links to the source.
Written as at 11th May, 2020.

Here is the Australian roadmap, which will guide the states to make their own directions:

https://www.pm.gov.au/sites/default/files/files/covid-safe-australia-roadmap.pdf

Queensland

Currently, 5 people can be involved in a wedding. This consists of the couple, the celebrant, and the 2 legal witnesses.

From midnight 15/5/2020, there can be the couple, the celebrant, and up to 10 guests (totalling 13 – lucky for some!)

Here is the roadmap from Qld Health:

https://www.covid19.qld.gov.au/__data/assets/pdf_file/0016/127150/DPC7309-COVID-19-Restrictions-roadmap-A4L-v27.pdf

Along with the Frequently asked Questions which gives further information:

https://www.covid19.qld.gov.au/__data/assets/pdf_file/0019/127234/Easing-restrictionsQA.pdf?fbclid=IwAR0kgMAszRUKMW9ehYxur4X5IdwJf7ggJAm9S8nTkgrIVIjsNd_kVdmHVV4

New South Wales

Currently, 5 people can be involved in a wedding. This consists of the couple, the celebrant, and the 2 legal witnesses.

From 15/5/2020, weddings can have up to 10 guests plus the celebrant and the couple.

Here is the link to the NSW information:

https://www.nsw.gov.au/covid-19/what-you-can-and-cant-do-under-rules

Victoria

Currently, 5 people can be involved in a wedding. This consists of the couple, the celebrant, and the 2 legal witnesses.

From midnight 12/5/2020, weddings can have up to 10 guests in addition to the couple and the celebrant. Like Qld, this will total 13.

Link to information about weddings in Victoria 

https://www.dhhs.vic.gov.au/stay-home-restrictions-coronavirus

ACT

Currently, 10 people, including the couple, can attend a wedding. The celebrant, photographer, and any staff are in addition to this 10. 

Here’s the link:https://www.covid19.act.gov.au/faqs/faqs-changes-to-restrictions-8-may-2020#Weddings

South Australia

From today, 11/5/2020, weddings can have 10 people, plus the celebrant and staff.

Here is the link to the SA wedding information:

https://www.covid-19.sa.gov.au/__data/assets/pdf_file/0012/195879/FINAL-DPC_RoadMap_FINAL.pdf?fbclid=IwAR03VckSDM1H5vywV6RDHKyoE3nAh6M1zjfmSZ4aQyYH_tqJ6ZO-HO7iTa8

https://www.covid-19.sa.gov.au/__data/assets/pdf_file/0011/196184/FAQ-Road-Map-Step-1.pdf

Western Australia

Currently, Western Australia has more relaxed conditions than the Eastern states, and has up to 10 guests permitted.

From 18/5/2020, 20 guests will be permitted for indoor weddings, and 30 for outdoor weddings.

Here is the link for information about wedding numbers in Western Australia

https://www.wa.gov.au/organisation/department-of-the-premier-and-cabinet/covid-19-coronavirus-wa-roadmap

Northern Territory

Currently, gatherings of 10 are suggested, but the number can be increased if social distancing is adhered to.

Here is the link for NT wedding advice:

https://coronavirus.nt.gov.au/__data/assets/pdf_file/0007/819034/roadmap-new-normal.pdf

Tasmania

Currently, 5 people can be involved in a wedding. This consists of the couple, the celebrant, and the 2 legal witnesses.

From 18/5/2020, up to 10 people can be involved in a wedding.

Here is the Tasmanian wedding link:

https://www.coronavirus.tas.gov.au/families-community/roadmap-to-recovery