[TK2005]

As a Brit National does anyone know if I need to arrange an ETA before entry to Australia?

Is this something I need to do in advance or can I just complete a form on the plane?

Cheers!

[martyn]

ETA? Estimated Time of Arrival?

I don't think you need to arrange that. You'll probably need to fill in the immigration form which they should give to you on check-in or on the plane.

[PDLM]

Electronic Travel Authorisation.

Yes, you need to arrange it in advance. If you don't have it then the airline will refuse to let you board. Either your travel agent can do it (they may or may not pass the charge on to you), or you can do it yourself online:
http://www.eta.immi.gov.au/
Note that British Nationals (Overseas) cannot apply through that site - only British Citizens.