In all fairness the use of the description dangerous, may depending on how you look at it apply.
Ability to spread faster is a kind of danger. 
More severe disease is a different danger.

The spike change N501Y is common to UK and South African, though mutations elsewhere differ.

So concern is that what we 'may' be finding out about UK variant disease severity as well as spread may apply to this one, but too early to say.
Of course winter, and the UK Christmas relaxation, New Year, and apparently secret raves, travel, weddings and such could drive spread too.
Decline in UK seems to show UK one is not immune to masking, distancing and restrictions.

https://www.abc.net.au/news/2020-12-31/what-are-the-south-african-and-uk-variants-of-covid-19/13022430

""
South Africa and the other from the United Kingdom, have both shown an identical mutation in that same spike.

Professor Gilda Tachedjian, president of the Australasian Virology Society, said it was referred to as the N501Y mutation.
""

From public health view its probably good to be cautious, by the time we know absolutely its a bit too late.

Hopefully the tracing and knowledge of likely origin allows us to squash this.

Aussie will be watch re our bubble, and hopefully we can boost their confidence with our response.