Your question is the subject of a 1974 paper by one R. A. Heath of the New
Zealand Oceanographic Institute. The paper is titled, quite aptly, "SURFACE
OSCILLATIONS OF LAKE WAKATIPU, NEW ZEALAND." A PDF copy of the paper can be
found on the Royal Society of New Zealand website.
The above paper goes into detail on the causes of the oscillations of the lake,
and the abstract of the paper may be found on the same site.
"Spectral analysis of lake level records from Lake Wakatipu reveals significant
energy in oscilations with periods near 52.0, 26.7, 18.5, 15.0 and 10.0 min.
Comparison with periods calculated by numerical integration indicate that these
periods correspond with the fundamental mode seiche for the whole length of the
lake and the next four modes. The most energetic mode is not the fundamental
but the second, with a period of 26.7 min, and this unusual situation is
explained by the method of generation in this dog-leg shaped lake."
The website of the School of Ocean and Earth Science and Technology (SOEST) at
the University of Hawai'i at Manoa provides more information on seiches.
"In simple terms, what is a seiche?
Every enclosed body of water has a number of natural resonances. If you sit in
a bathtub part full of water and rock back and forth you'll find that at the
right period (about a second) you can easily get the waves to grow until they
overflow the bath. The resonant oscillation of the water is a seiche."
If you need any clarification, please feel to ask.
Google search on: "Lake Wakatipu" "New Zealand" seiche
Looking Forward, denco-ga - Google Answers Researcher