As far as I know, the continuous service can be part time or full time: the legislation doesn't discriminate. However, some organisations may attempt to pro-rata your length of service and imply that you needs 12 months of FT work......but this isn't the case.

In your situation, with a total service of 20 months, you should be fine. But I am not sure of the stance that your rmployees will take on the fact that you were casual for a portion of hte time, so check with your manager and/or your HR Dept if you have one.

Hope this helps,