  A new astronomical observatory in southeastern Indonesia is currently under
construction. This Timau National Observatory will host a 3.8-metre telescope
for optical and near-infrared observations. To support the operation and
planning, the characterisation of the site needs to be appropriately performed.
However, limited resources and access to the site hindered the deployment of
instruments for comprehensive site testing. Fortunately, \emph{in-situ} sky
brightness data from the Sky Quality Meter (SQM) have been available for almost
two years. Based on the data acquired in 470 nights, we obtain a background sky
brightness of $\mu_0=21.86\pm0.38$ magnitude/arcsec$^2$. Additionally, we
evaluate the moonlit sky brightness to estimate the atmospheric extinction
coefficient ($k$) and level of scattering on site. We find an alleviated value
of $k=0.48\pm0.04$, associated with a high atmospheric aerosol content. It is
considered regular for an equatorial area situated at a low altitude
(${\sim}1300$ masl). By analysing the fluctuation of the sky brightness and
infrared images from \emph{Himawari-8} satellite, we estimate the available
observing time (AOT) of at least $5.3$ hours/night and the yearly average
percentage of usable nights of $66\%$. The monthly average AOT from SQM and
satellite data analysis correlate with $R=0.82$. In terms of the monthly
percentage of usable nights, the correlation coefficient is $R=0.78$. During
the wet season (November-April), the results from SQM and satellite data
analysis deviate more significantly, mainly due to the limited capability of
Himawari-8 in detecting fragmented low-altitude clouds. According to these
results, we expect Timau to complement other observatories greatly.
