@southerncross, in Australia there is at least one
supplier of certified safety glasses with bifocal inserts.
In my experience these glasses are much stronger and have lasted longer than the bifocal glasses I bought at local orienteering events, which had a plastic frame prone to cracking, and then a lens falling out. All of the yellow electricians tape I wrapped on the non-safety glasses to hold them together made them easier to find when they fell off.
The cost for a single pair is similar by the time you allow for postage ($A25-$A30).
You may be able to get the safety glasses at industrial safety suppliers, may even be tax deductible if they are also work-related.