Two young men critically injured in a house fire near Dunedin had been sniffing gas from an LPG bottle before an explosion which started the blaze, authorities say.

A Fire southern communications spokesman told NZ Newswire that the men, aged 18 and 19, were inside the house in Mosgiel, west of Dunedin, inhaling gas from the bottle when it suddenly exploded about 2pm on Monday.

The men, the only people inside the house at the time, were rushed to Dunedin Hospital with serious burns.

On Monday evening the pair were in a critical condition in the hospital's intensive care unit, Senior Sergeant Tania Baron told NZ Newswire.

The walls, ceiling and windows of the house were extensively damaged and tiles on the roof were blown off in the blast.

This is the second explosion in New Zealand on Monday.

Up to 50 firefighters rushed to the scene of a blast, also described as "significant", at the Taylor Preston meatworks in Ngauranga Gorge in Wellington about 12.30pm.

No-one was injured and it appeared the explosion had come from a dryer, a Fire central communications spokesman said earlier.