It's the nymph (youngster) of what is commonly called the Bronze Orange (as in citrus) Bug, musgraveia sulciventris. It's a pest of citrus groves in your part of the world.

Since you have dozens of them, I'd dispatch them soon, before they develop wings (which they don't have YET) and are more difficult to grab.

Thank you very much Rhizo_1
I did not expect such a complete and rapid response and from half the world away!
That's the beauty of forums like this.
Thank you again and many regards from a fellow Leo, John

You are most welcome.

Following Rhizo 1's great advice I googled these little critters and found a geocities web site run by a Peter Chew that explains these are nasty beggars - a form of stink bug. I can't tell you the website URL as posting rules prohibit it but the site shows all the stages of development and explains about the stink and marks left.

An organic kill method from a couple of other sites is a mix 1 part vinegar, 3 parts water and 5gms of soft soap altho I'm going to use white oil instead of soap. Best to use face mask and gloves and use a long shaft sprayer

I did some further research, too, and found that this insect AIMS for the eyes when releasing its secret weapon!

I thought I might bring people up to date on how things are going with the stink bugs. The first thing I should mention is it was some days before I got to them by which time some had become adults.
The second thing was that once I attacked with the vinegar and water mix (1:3) I could not believe how many there were! Suddenly there were seething masses of them that I couldn't see until they moved.
Intrepidly I pressed on behind full face mask usually used for brush and whipper-snipper cutting, gloves and long boom sprayer. Many fell to ground instantly, only 1 adult flew away.
Waited several days and checked them out and I would say got 80% of 4th instar which rhizo 1 identified and 30% of adults (dark black/brown) but curiously I didn't spot any final instar on the shrub. Incidentally its a kaffir lime and the shape is basically a 3 foot diameter globe. Fifty yards away we have lemons, mandarins and cumquats with NO infestations. And almost overhanging the lime a Valencia orange tree 20 feet high in the same NO bugs condition. Go figure!
Back to the critters. Follow-up attack was pyrethrum at recommended dosage with white oil as a spreader. This made serious inroads and basically killed the remaining nymphs, my guess is survivors are ones which managed to avoid getting spray right in face, and all but a dozen or so adults. I think that one more dose of the pyrethrum will see the end of them but boy! they are tough. Both spray loads were 2 litres ( ~8 cups) which is pretty fair dousing given size of little tree.
The second pyrethrum attack left all but a dozen dead so rather than mess around I hit them with an aerosol insect killer and bingo, all dead.

These things are so tough in future I wouldn't bother with vineagar mix and would use pyretrum straight up.