Cut off the affected areas. If the roots are healthy it should recover. You may want to take a few cuttings from healthy parts of the plant as a precaution, but it should be okay.

Cyclonenat,
Where are you located? Just curious! I agree with Rennet.

Natalie is somewhere in New Zealand, I believe, Kelly.

I am in Invercargill New Zealand right at the bottom of the south island

I would recommend moving your Hoyas to a warmer spot as those temperatures are far too cold and over time you will notice more and more plants suffering. There are a few Hoyas that can take light frost but they are not common species. Warm growers will suffer below 15C and most others below 10C. You might not notice problems right away but over time I think you will start to loose plants.

Check the SRQ Hoyas site for the temperature tolerance chart that David Liddle compiled.

Mike