The plastic tanks
should be no issue - according to the same rule
of the tank having a clear certificate label. They are not an American invent either as we had used them ever since the 90ies in Sweden
The steel tanks
are not 'cheap' everywhere. Here, in Spain
(Canary Islands), the 6 Liter tank cost is around 30 Euro (45 USD). Cheap
? Well, not really as they have 4 different LPG gas companies and re-sale on leaving varies depending on whether your original provider serves this specific area. NB 30 Euro = 35 Liters of gas (propane and butane available).
My bad experience with NZ was 2004/5 when Opua and Auckland
refused to fill my Swedish tanks certified autumn 2003. Whangarei did not have this issue, but I do not think it a good deal to go 10 miles up a tidal river just to fill my LPG tanks.