Churning from MyNetFone to iiNet - 3 days downtime due to "ported line"

Hi guys,
I've just had a call from iiNet with regards to my phone line, which apparently is a "ported line" which will require a three day downtime while the transfer is occurring from my current provider MyNetFone. I'm trying to work out what this actually means and why it will take so long when I wasn't aware when I signed up to Ace Communication from a deal posted on OzBargain (who then sold their serviced to MNF) that anything different would occur with regards to my line. I have changed providers many times as prices changed over the years, and it has never taken more than a couple of hours to churn my line over, so I'm wondering why this time it is so different? Does anyone have any insight and if there is anything I can do?

Thanks!