I think you need to look at the benefits of PV and the cost of the heat pump as two separate issues.
I built a pool last year, with a Pentair 140 heat pump. I had preexisting 6.38kw PV system on my home, but was still paying about $1300 annually for the power I used in excess of that. My cost per kwh is ~22 to 38 cents an hour depending on season and time of day -- so fairly expensive here in California. We have net metering, so I get an annual bill for the net difference between what I generate and what I consume. If I generate more than I consume, the power companies buys that amount at a very low rate, like 2 to 4 cents a kwh.
As far as I can tell, my heat pump runs at about 7.5kw, or 7.5 kwh for each hour it runs -- which is about 3 hours a day during the months I'm extended my season. So around 675 kwh per month for maybe 3 to 4 months of the year. As I was already consuming all my PV power and more on air conditioning and other power needs, the annual cost for the heat pump (back-of-the-napkin-math) was maybe as much as $800 per year when factoring in the pool pump as well.
I was able to add a second PV system to my home for about $11,000 after tax credits for a 5.5 kw system, just completed this past December. If I have all my math and power estimates right (I hope and think I do), this should be sufficient to bring my annual electric bill to zero, even with the heat pump. So it will cover the $1300 a year I was spending on power before previously, plus the additional $800 annual heat pump cost - total of $2100 per year. So in 5.25 years, the panels will pay for themselves (I financed the panels for 7 years at 6.5%, so let's call it a 6 year payback after the interest costs). After it pays for itself in 6 years, it's like getting $1300 off my power bill plus the free heat. It's actually even better than that, because here in California we have PG&E as our utility, which has just declared bankruptcy because it caused billions and billions of dollars in damages from wildfires caused by their equipment, so our rates are going up for sure. Now I won't have to suffer those increases.
So that's the kind of analysis you should be doing. What's the power consumption of the heat pump going to cost? What does your overall power consumption and cost per kwh look like? How much will your PV system cost? A couple of tools that might be helpful is the solarpaneltalk.com forum, and a tool to estimate your actual power generation called PVWatts at
PVWatts Calculator (it's a US tool but I think covers Australia).
