It might help to have a picture of the panels specification label but I am guessing these are 72 cell panels ?
In that case VMPP will be around 36V and IMP about 5 Amps.
Given your extraordinary nany state laws in Australia and this limit of 120V the maximum number of panels you can series connect would be three giving Vmp of ~108V.
You dont say what heater you are considering but if you use this calculator
http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm its fairly quick to find how much power you can expect from a given heater at MPP.
Without an MPPT controller to match the suns output to the fixed heater load your performance will be quite poor, the problem is at lower insolations the heavy fixed load will drive the PV voltage down so the power will be way below MPP.
I think you should take up Codebirds offer or just take the $400
By the way as indicated switching medium DC voltages without correctly rated switches is a fire hazard, especially avoid built in switches or thermostats of household goods.