What is the National Cabinet?
National Cabinet is a co-ordinating body, made up of the Prime Minister, State Premiers and Territory Chief Ministers.
In response to the COVID-19 pandemic and its impact across Australia, the Prime Minister initiated the National Cabinet to coordinate action and ensure prompt communication between the Australian, state and territory governments to manage this national crisis.
As a co-ordinating body, the National Cabinet seeks to ensure agreed national action, preparedness, and response measures are implemented as consistently and effectively as possible, nation-wide – considering the health, economic, society and public safety impacts of the virus.
What happened to COAG?
The National Cabinet has replaced the Council of Australian Governments (COAG) – dissolved as of 29th May 2020. The membership of National Cabinet is similar to COAG with the Prime Minister as Chair, and State Premiers and Territory Chief Ministers as members. As Chair, the Prime Minister generally provides a brief to the media on outcomes shortly after the meeting.
Is National Cabinet more effective?
Under COAG, the federal, state, territory, and local bureaucrats would generally create the agenda ahead of time and respective leaders (PM, Premiers and Chief Ministers) would meet to discuss what was listed on the agenda. Prior to the COVID-19 pandemic, COAG would only meet twice a year to manage matters of national significance or matters that need co-ordinated action by all three tiers of Australian government, however it was notorious for becoming bogged down in process and administration.
Given the dynamic nature of a pandemic, a more efficient and direct body was necessary for coordination of government response. National Cabinet enables the leaders to bring priority concerns which can be discussed directly with other leaders, before agreeing to a solution or coordinated approach which the Commonwealth, States and Territories then implement within their own jurisdiction.
Can’t the Prime Minister tell the Premiers and Chief Ministers what to do?
Because Australia’s federal system was created by the States, the Australian government only has certain responsibilities and legal powers. The Australian Constitution highlights that, “…any powers not specifically granted to the Commonwealth in the Commonwealth Constitution are ‘residual’ and therefore exclusive to the States.”  If you’d like a list of the legislative powers of the Federal Parliament, see Section 51 of the Commonwealth Constitution. 
Coordinating arrangements negotiated through National Cabinet to deal with national emergencies are not only desirable, but necessary, because no single government has the capacity or legal authority to deal with all facets of large-scale emergencies.