Are plastic storage drawers from bunnings or mitre 10 mega safe to keep fish in as a temporary thing or will it leech plastic or something? Can you safely put a heater in it? Has anyone keep fish in them before?

Yes, I have kept fish ( guppy and gourami fry mostly) in clear / transparent 'hobby box' type containers for a year or more, with heaters, quite safely. Also used some of the darker plastic tote boxes outside for mini fish-ponds. I washed mine out well with detergent and rinsed thoroughly several times to make sure there were no manufacturing residues. I suspect some of the lower-grade plastics with bright or fluoro-colours might be a bit dodgy, but clear plastic, food-grade plastic or better quality plastics I think are pretty much fine. I have fairly neutral water here, acid water conditions might be different perhaps?

I would make sure they are "food grade".

I use one those ones from the Warehouse with small wheels on the bottom as a quarantine / hospital tank, haven't had any noticeable problems. Cold water though & if I used a heater I would make sure it was kept away from the plastic (a lot of plastics soften at 60C). Also only keep it  2/3rds full to prevent any bowing of the sides.

Thinking that getting a tall one is going to be a waste of time since it will just bow does 55 liters sound like a good size?

http://www.3reef.com/threads/reef-safe-plastics-leaching-myths-facts.57828/

I also bought and used the clear ones on Warehouse, but no more than 60L.  I used them mostly with fry and juveniles.  With heater and no protection.

The late Alan Wakelin, killi breeder of renown, kept many of his in plastic drawers, all nicely labelled. I think he heated the fish room, not the drawers themselves.

I recently cycled a sponge filter in a clear(ish) lidded 60 litre Taurus one from The Warehouse and have some gouramis in it now while I get their tank set up.  The heater is stuck to one long side horizontally using the suction cups; even when it's heating, I can't really detect any temperature difference between the plastic beside the heater and the rest of the bin, so it's certainly not going to melt.  I cut a couple of slots in one end of the lid with a hot knife for the air line and power cords.  Think I put about 50 litres in there, no substrate.  The sides and bottom are bowing but it's holding together fine (I wouldn't use one that has seen any UV though).  I just rinsed it out with hot water in the shower.  Fish haven't died yet :-)