That value is largely based on the rarity of the stamps stuck on it, but also because of its use.

These tags, and similar items described as parcel labels, are now considered an important part of postal history. They are sought after by increasing numbers of philatelists.

Prices paid for these started to rise about 15 years ago, peaking in about 2007 before the global financial crisis came along and slammed on the brakes.

On March 29, the largest collection to appear in recent times was listed at a Melbourne sale by Phoenix Auctions. That $5000 tag (pictured bottom) was passed in but the majority of the others, 21 out of 28 listed, sold for around their estimates.

The best price paid was $1980 (including buyer's premium) for a 1939 tag featuring a rare £2 grey-black and crimson Kangaroo and map stamp (a tiny printing flaw adding to its value), along with three lesser Kangaroos and three other stamps on the bottom row.

According to the catalogue, "the tag is believed to have paid postage for a 27lb 9oz gold bullion shipment from Melbourne probably to California for refining …".

A few minor blemishes affected its total value. The estimate was $2000.

"These appear from time to time," says David Wood, Phoenix's managing director. "They were selling for more two or three years ago, but are still trending upwards. The number of people collecting them has definitely increased."

For Kangaroo stamp collectors, also increasing in numbers, this is a way to buy the larger denominations in a compact format.

Use is another determining factor. Tags are colourful and quirky items that ooze history. The one used for sending gold to California is one example.

Another label sold recently by Phoenix was from a parcel sent to the local council containing the scalps of wild dogs.

The rarest of all are official post office labels for mailing queen bees in special containers made of edible wax. Only two examples are known to survive.

Back in 2007, I visited the luxurious Sydney home of Kevin Nelson, a former regional manager at ANZ bank who, in retirement, has become a kind of international stamp superstar.

His special interest is in displaying stamps on the international exhibitions circuit. He then sells the award-winning displays to finance new projects. He likes to describe himself as an investor/exhibitor.

As a former banker he is well aware of the investment value of stamps. His 2007 exhibition, Australian Postal History (1901 - 1940), was based on the kind of parcel tags and labels shown here, many never before seen in public. "It will raise a lot of eyebrows," he predicted then. And it did.

Nelson is seen by David Wood as the man most responsible for popularising this niche in the philatelic market.

As Nelson said at the time, millions of these tags would have been thrown away. But according to the logic that if anything exists it will be collected by somebody, a small number was kept. Many of the stamps were soaked off and placed in albums, but the few that were kept intact are now rare and valuable.

The most expensive in Nelson's collection was a rare two-shilling ''first watermark'' stamp on a label, plus details to show that the parcel was sent from Tasmania to Switzerland in 1913. In 2007, its value was estimated at $10,000 plus.

Much can be gathered from the scattered pieces of information stuck on a parcel.

''Deceased Kits'' labels indicate the package contained the final possessions of a soldier killed in battle in World War I. These are now very rare. Nelson had one of these plus one of the two bee labels known to survive. This bee flew from Toowoomba to Melbourne in 1938 - by air mail, of course.

Seven years ago, Nelson estimated the auction value of both these treasures at $1000 each. Those prices would have at least doubled or tripled during the past seven years.