Operating income has increased from $31m in 2015 to $44m in 2018...

Operating income has increased from $31m in 2015 to $44m in 2018 - close to 50%
Distributable income per security has increased from 10.2c in 2015 to 13.1c - close to 30%
Net asset value in 2015 has increased from $1.33 in 2015 to $1.97 in 2018 - close to 50%

The SP at 30th June, 2015 was $1.53 and is now $ 2.79 - close to 80%

Some of the extra price increase is probably due to the fact that solid growth is evident now and was less evident in 2015.
Some of the extra price is due to the buying power as investors read about dubious economic conditions and head for safer ground. Then, the thing is that most of the REIT's are retail based and the punters are losing interest in these assets so there are fewer safer grounds to turn to and ARF is, therefore a bit of a standout.
SYD yield is around 4.77% while ARF is around 4.59%
I've now run out of reasons to support the price other than that the ALP, presuming they attain power soon, may provide even more support for Child Minding centres.

I don't think, after all that, the price is too far off reasonable - maybe just a little bit.