killing wisteria regrowth
I have tried killing this pest for the last two years with pure Round-up, diluted Round-up and Blackberry Poison. Ive sprayed the stumps, the roots and sprayed the leaves which has killed the plant leaves, but if it rains when seem to come up even thicker. Ive dug roots out and treated the ends with diluted Round-up, but still they surface. Whats my next move.
The methods you describe for eradicating wisteria should have been effective. But lets review some variables that could have been at play.
There are different formulations of Round Up with various percentages of the active ingredient, glyphosate. An effective cut stump treatment is with approximately 25% Glyphosate or more, so if you are using a RU concentrate with 48% a.i. then dilute it with water 50:50 or use it straight. For cut stump treatment the most effective is to treat immediately after making the final cut, don't wait even a few minutes. Cut and treat only actively growing plants, not during dormancy.
Lacking a fresh cut stump, wait for substantial re-growth and spray the foliage with RoundUp ProMax 4% to 7% solution. Or there is another very effective product used for control of woody shrubs and vines, Pathfinder II. You can spray or paint the straight product on the growing stems and it will penetrate and translocate throughout the plant tissues. Not sure of a source in Australia, but here is an example:
Also, consider that the re-growth may be seedlings sprouted from seeds in the soil. If so, then keeping up with spraying or grubbing of active growth as it appears may be the only way.