I just switched over from Virgin to Optus using an offer that they'd waive all fees remaining on the Virgin account. I just got billed by both Optus and Virgin, and the Optus bill I can't even view (it is not available under the download bill section of the website or the app. The Optus service has only been active for 5 days, I don't understand how they've charged me a whole month's fees. The woman on the phone also said I'd be charged again in the first two weeks of November, which is not a month after this first mystery bill (presumably, I'd get the bill for that soon). Apparently there's also a preservice charge on a post paid bill? But I was not told this and I can't find anything about it online. How do I go about actually seeing the bill and getting answers?
Optus change over billing can be fraught (and look messy). As a general rule OPtus will only look / fix billing issues after a bill has been printed.
You should be able to contact Optus directly (Facebook? Livechat? The Optus App?) and get them to sort your account difficulties / send you a bill etc.
ONce you have the bill you can work out what is needed to do. FWIW Optus routinely charge first and last months 'rent' in advance. This is useful as it means they don't need to disconnect you as soon as you pay a bill late. You won't be over billed over the term of your contract.
The Virgin refund might happen automatically or you might need Optus to handle that manually (they may be waiting for the final bill from Virgin)
All up I'd not be too concerned and it should all settle down over the first two bills. Its just a pity it has to be this messy.