No, it's because they have a captive audience now.
The LPG rebate made thousands and thousands of people who would normally never consider LPG to convert thier cars...hell, we almost did...glad we didn't.
Now that heaps more people have LPG powered vehicles, it's safe to raise the excise as much as they like to charge extortionate prices for LPG.
We export the stuff in bulk for a few cents a liter, yet charge our own public a price waaaaay out of proportion to what the stuff costs to make per liter. LPG only makes sense when it's half the price of petrol or less. It
used to be down around a third the price, or even less.
And yes, diesel used to be dirt cheap...those of us who've been driving for up to thirty years will remember the way the pricing worked. When I started, I vividly remember petrol being 36 cents per liter, diesel was only about 18 cents a liter (as said above, trucks, some old fourbies, and the very rare European car would use it), and LPG was only about 8 to 10 cents a liter. I remember the LPG price as I briefly toyed with the (at the time) novel idea of converting my Valiant Charger to LPG, as I was spending over $50 a week on petrol...
As more people take up the use of LPG, the excise will rise to slap the cash cow on the bum. Same with E10...it's artificially kept lower than normal unleaded by taxpayer subsidies.
However, wait until some people get thier way and normal unleaded is phased out (because deliberately making your car get
worse economy is somehow better for the environment...
), and once people are a captive audience, we'll see the subsidy removed and the
true cost of the stuff will be revealed.
Same with LPG conversions...there was a massive take-up of that idea, but the popularity has dropped off now that the price difference now makes it far less economical than it was before. The sums don't add up on each and every car...it works for some people and the payback is pretty quick, but it depends on the quality of the conversion, and some were pretty dire. If for your particular vehicle the economy on LPG drops below a certain percentage of unleaded and you're only saving a small bit each week (which you could do by modifying your driving habits or keeping your car well serviced anyway), you may as well not bother and just keep running unleaded.
Only blind freddy never foresaw the excise on LPG being raised.