My wife and I are planning to spend five months in France next year. We are Australian and understand we need visas for a stay of longer than 90 days. Can we avoid having a visa if we break our stay by visiting another country, eg Spain or Switzerland, and then re-entering France a few days later? How long would we need to absent ourselves from France? Thanks for any advice on this subject.

No, both Spain and Switzerland are in Schengen. The rule is 90 days out of 180. So after 90 days you would have to go for another 90 days to a non-Schengen country (UK for example) before you can return to Schengen.

Elina, thanks very much for that. It confirms what I suspected. I must now make a start on assembling all the paperwork (and money) needed for our visas!

As you have found out, to stay 5 months in a Schengen country, you need a long-stay visa. It's not an easy visa to obtain for France, as all applications are sent to Paris for processing. You need to provide, among other things like proof of funds and onward or return ticket, health insurance coverage, medical certificate (e.g. being free from contagious diseases, mental illness and drug addiction)and usually a police criminal record clearance.
Once you register your residence at a local mairie or hotel de ville, you are allowed to visit other Schengen countries, officially for 90-in-180 days but in effect as long as your visa remains valid.

Thanks Alec. Hope they will look kindly on a retired couple planning to spend a couple of months in two language schools and most of the remainder living la vie Francaise in a small town in the Midi.