As per posters above, there's no problem with getting imports into the country, other than the need for them to test negative and spend a couple of weeks in quarantine. Clubs won;t be worried about the couple of thousand that will cost them, per import. Although it may mean imports are less likely to be game ready when they come out of quarantine. So the point above about getting import selection right is a good one.
A bigger threat to NBL is the risk that state and territory border restrictions may still be in place come December. Victoria will be in lockdown for at least another month and presumably will take a month or so after that to ease back restrictions. That'll take us to the end of September. Borders with other states and territories might open up a month or so after that, depending on case numbers in Victoria. So end of October. That'll be only 5 weeks before the NB: season is due to start. That's a best case scenario and presumes the NSW outbreaks don't explode like Victoria's have. It also presumes no further major outbreaks arise. It also presumes rapid re-opening of borders if and when the current outbreak is controlled.
If you think the above is too pessimistic, then consider this. In the initial wave of COVID-19 cases, Victoria's average new cases per day peaked at 56 on March 27, then fell to 2 by April 28 (a month later). In early June, that average was around 6 per day and SA said it would reopen the border a month and a half later - i.e., reopening borders 12 weeks after Victoria's 56 per day peak. That decision was rescinded within 3 weeks, after Victoria's current outbreak began. In the past week, Victoria's averaged over 270 new cases per day (five times the initial wave's peak) and it's still going up.
I think the NBL should be nervous.