The rest of my street is now on HFC NBN but my house is an outlier, still waiting on additional work. Been waiting since May, I suspect it'll take until the next May.
With 3 people in the house, going from 3 mbit to 25 or 50 will make a pretty huge difference to us. Probably about as much as when we first jumped from 56k to 256k ADSL 15 years ago.
Second important question: do you have fiber to the premises, ie an NBN box on each house? If you do...
Remember that the NBN box (unless they changed it) has 4 data ports, each of which can be associated with an isolated ISP plan. Presumably your neighbor is only using Port 1.
If you're close enough, you could probably get away with a couple amplified 802.11ac dongles. A Pi3 or modded router could handle the Wi-Fi point-to-point setup at the remote house.
If that wouldn't quite work, point to point microwave links (which don't require a license, and are basically a pair of boxes that you stick on REALLY tall poles, and which have Ethernet ports on the back) are around $450 a pair. (For example, http://store.freenet-antennas.com.au/product_info.php?cPath=...)
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If you have fiber to the node (ie the street), I have no idea if extra connections can be added for a house NBN-box-style or what that looks like. It probably means provisioning an extra phone line, which obviously wouldn't work.
But you could viably rent the neighbor(s') actual bandwidth in this situation - in fact, if you can successfully describe and sell a "0% QoS" structure, where whatever bandwidth is not being otherwise used at any given moment is allocated to you, you could probably get away with paying very little. :P
You could also rent from multiple neighbors, too. Supposing an ideal scenario where you rent from 10 neighbors which have 100Mbps each (I know, I know...), you could get a VPS with decent bandwidth in a Real Country™, connect to the VPS from each borrowed connection, bond the resulting TUN interfaces together at both ends of the link, and hey presto - absolutely disasterous ping and jitter, but 1Gbps throughput at 1 o'clock in the morning.