Green’s Dictionary of Slang

flattie n.2

also flatty

1. a small, flat-bottomed sailing boat.

[Aus]Bulletin (Sydney) 21 Nov. 16/1: I was navigating a flatty up one of the numerous creeks that empty into the Cairns Inlet, North Queensland.
[NZ]J. Devanny Paradise Flow 23: He [...] rowed himself across the river in his own ‘flattie’ to his hut among the mangroves.
[Aus]K. Willey Naked Island 121: The boys lowered the flattie to pick him up. He was so exhausted they had to lift him aboard.
[NZ]McGill Reed Dict. of N.Z. Sl. 79: flattie [...] 3. A flat-bottomed rowboat, used in shallow water, possibly to fish for 4. A flatfish.

2. (N.Z.) a flat tyre.

J.H. Henderson Down from Marble Mountain 184: Returning, I collect a flattie, rain begins, drenched I reach work.
[NZ]McGill Dict. of Kiwi Sl. 45/2: flattie a punctured tyre; eg ‘She’s given the old dunga too hard a time on the corners, she got a flattie on the back left and flapped to a stop.’.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].