This may sound like a dumb question, but if I change my isp (I'm with Optus and going back to Telstra), does that mean that my e-mail account with them will be cut off?

Or is it like hotmail and Gmail etc.?

Mate, ISP-based email accounts disappear when you change ISPs. Web-based services such as Hotmail and Gmail are portable as they are not connected to your Service Provider.

I have changed a few times over the years and have lost a lot of emails each time. If you have any important ones, I would create a Hotmail account and forward them to it, obviously before you switch!

In the US many ISPs provide email forwarding services. In fact I think Congress mandated it to happen. Try asking your current provider if they have forwarding.

Thanks ozzz. Will follow your advice.