How ethernet over power adapters work
Ethernet over power adapters deliver internet over the existing power circuit in the house. They use similar technology to WiFi although high frequency signals are delivered through power lines, not air. Hence ethernet may be delivered to rooms where usual WiFi cannot penetrate. The first adapter connects to the router in one room. The second or more adapters are installed in other rooms. Thus, the internet is shared across many rooms without running wires. To make WiFi in the other room, you'd also need a WiFi access point after Ethernet over power adapter or power and WiFi adapter in one. Both ways are roughly the same price, but separate access points usually provide better WiFi speed and reliability.
Kits include two adapters. But you can install many interlinked over power adapters. Over the power adapters of the same brand usually work with each other. So you can add more adapters as you need.
Here is the list of a few popular Ethernet over power adapters we use.
TP-Link ethernet over power
PA411 is Ethernet over a power adapter kit of two. Probably the most popular due to the price. The advertised link speed is up to 500MBs but reaching 100Mbs in practice, which is still sufficient for most applications. $72
TP-Link over power + WiFi
WPA4220 is an ethernet over power and 2.4Ghz WiFi adapters that extend the router to other rooms AND create a WiFi network. This WiFi adapter has a bit worse WiFi range than a separate WiFi access point due to a small size internal antenna. It is perfect for a small room. The WiFi adapter also has two Ethernet outputs. Speed is the same as PA411. WiFi speed is up to 300Mbs, which is sufficient for a few people on NBN. $120
TP-Link 1Gbs over power + WiFi
The WPA7510 powerline internet kit, with a 1Gbps capacity, is ideal for fast NBN plans exceeding 100Mbps. It extends your router's coverage to additional rooms and establishes a WiFi network. This WiFi adapter also features a single Ethernet port. Powerline transmission speeds can reach up to 1Gbps, while WiFi offers 300Mbps on the 2.4GHz band and 433Mbps exclusively on the 5GHz band. $140
Netgear 1Gbs over power + WiFi
PLW1000 is 1Gbs over power and 2.4/5Ghz WiFi adapter kit that extends the router to other rooms AND creates WiFi network. Powerline speed is up to 1Gbs. WiFi speed is unspecified. $240
D-Link 1Gbs over power + WiFi
DHP-W611AV is a set of Ethernet over power adapters and WiFi access point in one. WiFi speeds are only up to 867Mbps on 5Ghz and 300Mbs on 2.4Ghz bands; hence the name AC1200. The maximum speed over the power is 1 Gbs. This is similar to the CAT6 cable. $280