Holden dont stock the breather hoses anymore (only the big main one on the filler neck).
Thank the lord for that..... whatever they used is the most useless thing I've seen in a long time...... The black stuff used on late 70/early 80's holdens is usually still intact if you ever see it, be it brittle if you try to bend/remove it, but it's usually still sealed and not missing like the crap clear/opaque stuff....
I took a piece of the old line with me when I last did this (and everytime before), forget what size they had in stock but the ID (as above 3/16" is fine, 5mm is the diameter of the pipes on the spare tank here) was close, just use some crc/wd40 etc to aid getting the new hose over the pipes.
The OD was larger but wasn't an issue, it's a prick of a job at the best of times, loooong handle pointy nose pliers are damn handy to get the spring clips back on the tank (and the hose back on).
The larger pipe from the tank to the filler neck has always been fine when I've tackled this, It's still a pain but easier to just do the smaller lines if you don't need to do the big bugger (still leaves sfa room between the top of the tank and the boot floor either way).... forget replacing the spring clamps with new hose clamps on the top of the tank as well (easy enough to replace them on the vapor liquid separator)....