Hi Jerico,
The password based login on NeCTAR instances is actually not yet functional (an issue with how the machine image was built) but it will be fixed pretty soon.
In order to ssh to the instance, you should use the key pair that was generated when you requested the instance and the following command:
ssh -i <path to private key> ubuntu@<instance IP>
Note that the private part of your key pair is available only the first time the key pair is created. If you did not download it from BioCloudCentral, just use NeCTAR's dashboard to delete the key called cloudman_key_pair and, when you start another instance, the key will be regenerated so you can download it then.
Glad to hear you're giving this a shot and please let us know what your experience with it is.
Cheers,