Gavin, there are a few around. Nutritech Solutions at Yandina send their soil samples to the New England University I think then have inhouse agronomists to decipher your test.
They also make a wide range of amendments all organic and sustainable.
The cost of any soil test will be around $300+gst for the better tests that tells you about all the minerals & pollutants.
If you pay for the full test you get the agronomist free of charge, otherwise that service will cost a little.
You ring them, organise your test and pay them, they send you the post packs already addressed to the lab. You follow their directions to collect your sample and post it off. In about 2wks the results are emailed both to you and the agronomist. If you buy the full test it will be about 12 pages.
I get a test quite regularly because I have such poor soil.
Perhaps I can help you if you have a particular problem?
What are you noticing about the health of plants?
Aphids like fleshy soft tips. Often that occurs if too much nitrogen was used or the plant didn't need all of the usual amount. Also if your compost is a bit green as in not fully matured or there is a lot of chicken manure in it, it can cause soft tips.
More calcium can sometimes offset by helping the plant make stronger stems.
Cucumbers prefer a soil pH of 5-6 which is a bit acid but they still need a fair bit of calcium.
There is a limit to how much and what type of calcium you can use, as it will usually make the soil more alkaline.
Gypsum will provide calcium in time without changing pH. Its a bit slow acting.
I dry and crush my eggshells for my compost also. Mostly for use by the worms that need the crushed powder as grit for their stomachs.
Egg shells will raise the pH of soil but it also gets used up fairly quickly too.
Generally, you would know the pH of your soil by doing regular tests, then check your plants to see how they behave in that pH.
The pH will tell you what minerals are available in what quantity under which moisture conditions.
Then the plants themselves will tell you what's lacking.
Have you got a pH test kit?