According to details that were provided at launch, it looks like BlackBerry 10 devices will launch in Australia at around the same time as the US, meaning that Australians should see BlackBerry 10 devices in March.
As to 4G, the specific data speeds will depend on the carriers. If your carrier offers LTE, then they should be releasing an LTE compatible version (my Z10 is connected to the Telus LTE network). If your carrier does not offer LTE, then you will likely connect at H+ or 3G in Australia.
I hope that info helps!
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