I recommend reading user1686's answer first for a general overview. I'll be going into a bit more detail on the NBN network.
Since we're talking about the NBN specifically, I'll be pulling from their Wholesale Broadband Agreement, especially the nbn™ Ethernet - Product Description 4.1, updated May 2021 and nbn™ Ethernet - Product Technical Specification 4.1, updated May 2021.
NBN basic background
First, let's start with the Product Description to get some basic information:
The nbn™ Ethernet Product ... is an Ethernet-based Layer 2 virtual connection that carries traffic between a UNI used to serve a Premises and a POI
An Access Virtual Circuit or AVC is an Ethernet-based Layer 2 virtual connection on the Fibre Network, FTTB Network, FTTN Network, FTTC Network, HFC Network, Wireless Network or Satellite Network that carries RSP traffic to and from a UNI used to serve a Premises.
The User Network Interface or UNI is a physical port to which nbn supplies nbn™ Ethernet in respect of a Premises.
NBN Information Rate (Ethernet frames)
Then the Product Technical Specification has "§ 2.2.2 Bandwidth Profile Parameter Considerations", which describes how information rate limitations are applied:
All Information Rate limitations, including as set out in this nbn™ Ethernet Product Technical Specification, are enforced at the NNI interface between the RSP and the nbn™ Network.
Where the bandwidth profile is equivalent to or greater than the negotiated Line Rate, a degraded useable payload will occur.
The Peak Information Rate for nbn™ Ethernet is calculated on Layer 2 Ethernet service frames, over the series of bytes from the first bit of the Destination MAC Address through the last bit of the Frame Check Sequence. IEEE 802.3 physical-layer fields such as the preamble, start of frame delimiter and inter-frame gap are not included in the Bandwidth Profile.
Unfortunately, further detail of what exactly is in those Ethernet frames is in separate Network Interface Specification documents that do not appear to be publicly available. If we step back to the NBN Co Ethernet Bitstream Service Product Technical Specification 2.12, updated July 2017, no longer in force, we can get some insight before they removed them from this document:
§ 3.1.5 AVC/CVC Service Addressing Mode A uses a two-level VLAN addressing scheme at the NNI, which is compliant with IEEE802.1ad (Provider Bridges) to identify individual 1:1 AVC and CVC services.
The IEEE802.1ad S/C-TAG adds an additional 4 bytes overhead each (total of 8) to the Ethernet frame. Of course, we are making the assumption that this is still the case.
Ethernet payload additional overheads
This more or less follows the "internal overheads" as described in user1686's answer. Note that whether PPPoE is used depends on the RSP; the NBN does not care what goes in the payload.
Additional line rate limitations
Aside from the AVC limits, there can also be limitations due to the User Network Interface (UNI), i.e. the specific technology used to connect the user to the NBN.
These are generally limited by what is achievable, and any protocol overheads are not included in the information rate limitations (with maybe an exception for VDSL2 Ethernet over copper? see below). Specifically, any DSL overheads are not included in the advertised bandwidth nor checked by any shaping or policing -- but, of course, if your line rate is too low it will impact your bandwidth before any shaping or policing come into effect.
Note that there are two data categories of UNI:
UNI-D, covering the networks that provide an Ethernet port to the end user (incl. FTTP, FTTC, HFC, FW, Satellite). This provides a 10/100/1000BASE-T/TX port. On UNI-D, a user is generally limited by AVC only, unless they purchased a 1000Mbps AVC where they will be limited by the port speed (1000BASE-T for the entire Ethernet frame).
Note that there can be additional limitations on achievable bandwidth depending on the network. This is most apparent on Satellite and FW, followed by HFC.
UNI-DSL, covering those networks that use VDSL2 and require a VDSL2 modem (FTTN, FTTB). A user can be limited by the line rate, which depends on the quality of the copper phone wiring and distance to the node.
§ 188.8.131.52 For DSL services the Information Rate is limited to the lesser of the aggregate AVC bandwidth and the actual Line Rate on the UNI-DSL. Note also that for VDSL2 the Line Rate and Information Rate are subject to VDSL2 Ethernet over copper framing overheads as defined in the ITU-T VDSL2 specification G.993.2.
UNI-V is that is a voice-only product with very limited high-priority bandwidth (for VoIP) that is not available outside of FTTP and not supported by all RSPs. The UNI-V port is an ATA and provides only an analog phone connection to the user (VoIP/SIP is handled on the NBN/RSP side and not exposed).
Pre-mid-2020, NBN policed traffic at the advertised AVC speeds, which resulted in lower observed speeds on common internet speed tests (e.g. speedtest.net, which uses either WebSockets or HTTP depending on browser). user1686 has already calculated these overheads as, roughly, 4.5-8%.
Around mid-2020, NBN started overprovisioning the downstream. Some documents from around that time describe this as:
For Home Fast and Home Superfast, nbn network management policers will provide an additional 15% overhead allowance to the AVC TC-4 downstream PIR at layer 2. This will allow End Users to potentially achieve a downstream speed on their network approximate to numerical value of the published layer 2 wholesale downstream PIR where nbn infrastructure has capacity.
This 15% overprovision is enough to ensure that common speed tests (over WSS/HTTP[S] payloads) should never show less than the advertised downstream AVC rates due to NBN traffic policing.