Hey there Timmmay - we get them around Miramar, Wellington occasionally too. I believe it's a course start point for a harriers group. There will probably be single 'H's' every half block or so from that point as course markers. Someone would have marked out the course using a device filled with chalk dust that gets tapped on the footpath to leave a series of 'H's'. 

That's my deduction... can anyone confirm?

They go by the name of hash House Harriers, hence the Hs

Someone goes out and lays a different trail for each run...

https://en.wikipedia.org/wiki/Hash_House_Harriers