If I have a visa grant email from DIAC (Australian immigration dept) but do not have an Australian visa in my passport am I allowed to just travel with the grant email ? Will an airline even allow me on board ?
In general, Australia no longer does Visa stamps in passports. They are available, but only after paying a relatively high fee, and there is no real benefit to doing so.
All visas are tracked electronically against your passport, so when you give the immigration official your passport they will be able to see the details of your visa.
Technically you don't need the grant email, but I would suggest taking it just in case there are any problems.
Airline staff/systems also have access to Australian visa information, so they are able to confirm your status without needing to see a physical visa.
According to DIAC no you don't need to have a separate visa as long as you are traveling with the same passport as you have received a grant.