I don't believe the reason given. The SSPX can sponsor seminarians for visas. They can come to Australia but would have to quarantine for two weeks on arrival. That is not an insurmountable burden.
God bless,
Fr. MacDonald
It seems Fr. MacDonald is correct.
The following information is available on the official Australian Government Department of Health website:
Travel into AustraliaAustralia’s borders are closed. The only people who can travel to Australia are:
1) Australian citizens
2) Permanent residents
3) Immediate family members
4) travelers who have been in New Zealand for at least the 14 days before the date of departure.https://www.health.gov.au/news/health-alerts/novel-coronavirus-2019-ncov-health-alert/coronavirus-CÖVÌD-19-restrictions/coronavirus-CÖVÌD-19-advice-for-international-travellers
So I agree: "The borders are closed, so we must close the seminary" excuse does not wash.
However, the Society also noted that enrollment had been shrinking for some time (though I'm not convinced it had anything to do with travel restrictions related to CÖVÌD, when all that is required is a 2-week layover in nearby New Zealand, but who knows?), and that the low numbers made maintaining the seminary impractical.