Hi,
In order to enable load support, the inverter must be on.
The Magnum load support is NOT voltage support.
If we deal in amps--then VA won't be a consideration.
The Honda 2200 can do 18.3 amps, surge. Set the Magnum to 18 amps of input. (based on 120 volts--so you may have to check these numbers for Australia)
The Honda can do 15 amps, continuous. (1800 VA) (120 volts)
I found that as battery voltage degraded, the Magnum would eventually cause a 15 amp shore power breaker to trip, even with it set to draw only 13 amps.
If one wishes to "force" the Magnum to do voltage support, then drop the input amperage. At a campground in the summer I once, on a thirty amp feed, had to set the input amps to 12. That yielded 107 volts. The roof air was drawing 1900 watts. Fortunately, I had a large battery bank. The solar panels were helping out, too. The outside temperature was about 37 C. (98.6 F).
In hind sight, I believe I may have damaged the Magnum by "forcing" it to do voltage support.