Short answer : When entering using an ETA, you do NOT get a Visa stamp, as the ETA is electronic. You WILL get an entry stamp when you enter, and an exit stamp when you leave.
Longer Answer :
There are two types of "Stamps" that can be put in a passport - Visa stamps and entry/exit stamps.
Visas. As of a few months ago, Australia no longer issues physical Visas for most types of visas without payment of an additional fee. All Visas are recorded and tracked electronically.
Entry/Exit stamps. The rules of exit/entry stamps vary depending upon your nationality and how you enter the country.
For Australian passports, they have not stamped passports by default for over 5 years. If you ask they will happily do it for you, but they will never do it without asking.
For New Zealand and US passports, when entering Australia using SmartGate your passport will not be stamped when entering Australia. As you do not deal with immigration officials when using SmartGate there is no way to ask for a stamp to be given.
For all other passports entering and departing Australia your passport will be stamped.
It is expected that the use of SmartGate will be rolled out to citizens of additional countries over the next few years, at which time the number of countries that do not receive a stamp when using SmartGate will increase.
All entry/exits (including of Australians) are recorded electronically, as are any Visas that are issued automatically on arrival.