I have recreated the scenario on my test site which is v1.3.0.2 and still have the sam problem...

This is what it should look like according to my calculations;

Sub-Total: $10.00
Shipping (Flat Rate): $5.00 = $15.00
Discount Coupon (10%): -$1.50 = $13.50
Group Discount (10%): -$1.35 = $12.15
GST Tax (10%): $1.21 = $13.36
Total: $13.36

As you can see below the GST Tax does not re-calculate after the Group Discount;

Sub-Total: $10.00
Shipping (Flat Rate): $5.00
Discount Coupon (10%): -$1.50
Group Discount (10%): -$1.35
GST Tax (10%): $1.35
Total: $13.50

The GST Tax and Total appear to be wrong.
If I swap the order of the Discount Coupon so it is calculated after the Group Disount I get this;

Sub-Total: $10.00
Shipping (Flat Rate): $5.00
Group Discount (10%): -$1.50
Discount Coupon (10%): -$1.50
GST Tax (10%): $1.35
Total: $13.35

The Discount Coupon, GST Tax and Total appear to be wrong although Total is only 1c out and may be rounding issue.
If I revert to either the Group Discount or Discount Coupon on their own then it works fine;

Sub-Total: $10.00
Shipping (Flat Rate): $5.00
Group Discount (10%): -$1.50
GST Tax (10%): $1.35
Total: $14.85

Sub-Total: $10.00
Shipping (Flat Rate): $5.00
Discount Coupon (10%): -$1.50
GST Tax (10%): $1.35
Total: $14.85


INFO

The Zen Cart Version is 1.3.0.2, the Shipping and product are both taxable.
Both Disount Coupon And Group Discount have the following settings;

This module is installed
true
Sort Order
280
Include Shipping
true
Include Tax
false
Re-calculate Tax
Standard
Tax Class
--none--

Can anyone help or have you had a similar issue?

Cheers
Will