Petesin wrote:
They are only about 6 weeks old. I'm in north west queensland and it's too
hot to grow them over summer and autumn through to early winter is still
warm enough for them. We haven't had a day under 32 degrees since it rained
in mid feb and the one that is still healthy is growing quite well. I'm
thinking it may have something to do with mulch being too close to the stem.

We have the same problem here in Midwest .
if it is the mulch too close to the stem
Old cardboard milk or juice cartons
around the base of the plant stops the mulch and some bugs earing the
stems or rotting them .

Plastic 2 litre milk bottles also save the fruit trees from whipper
snipper ring barking

HTH