PowerSwitch

Our heat pump will blast out warm air ... sometimes.

However, it now seems to be switching into defrost mode quite a lot.

Our place is getting quite cold!

Is this a sign of a problem?

It MIGHT indicate an over sensitive defrost thermostat calling for defrosting before it is needed, but more likely is the consequence of cold and wet weather.
To extract heat from cold outside air will require that the outside air coil be operated at well below freezing point. This will form ice on the coil which then needs defrosting. If the weather be wet or damp as well as cold then ice will form more quickly.

The COP of a heat pump can drop alarmingly in very cold weather, and might be worth turning it off and heating with LPG or wood in such conditions.

Sounds like geothermal is the way to go in the UK, and indeed Australia too. Just like old Biff.

You get that throwing out cold air effect sometimes in Australia too. On very cold evenings.

One happy consequence of having a lot of damp air going over a cold coil when cooling the house is that we collect a lot of distilled water (smells horrible but the plants don't mind) that supplements the garden watering.