We have a Trek bike trailer (cheaper cousin of the up-market Chariot). Very cool and only weighs 12.5kg (I could lift it into the back of our station wagon when I was 9 months pregnant without a worry). DH has used it a bit with the bike (on footpaths only - you can ride on footpaths in Vic if "accompanying someone under the age of 12") but mostly it is used as our day to day pram (which it is perfect for). If you google Chariot bike trailers you'll find the Corsair which is very similar to ours.

A bike trailer is probably a better option than a seat if you are planning more than one child IMO....